I'm A Level1 Cleric, What Does That Mean?
ENTER THE CLERIC!
Who can be a Cleric? In AD&D, the PHB tells us that "Only humans will normally have Clericism as their sole class; thus they are the only Clerics with unlimited advancement in level."
This tells us how non-human Clerics are likely to be perceived in the world. Powerful non-human Clerics are practically unheard of, and thus, aspiring non-human Clerics would likely be taken less seriously. The use of miracles would be seen as a means to accentuate their other skills. For the Half-Elf, this may include Ranger, Fighter, or Magic-User abilities. For the Half-Orc, it may involve his abilities as a Fighter, Thief, or Assassin. These combination classes may belong to specific orders in a world with more commonly distributed demihumans (an order of Assassin/Clerics, for instance), but in a humanocentric game, it's likely that they would be begrudgingly or at best dispassionately trained, as they will never serve any Clerical order's highest ambitions.
The Cleric, at level 1, is called an "Acolyte." We know that Gary Gygax was a devout Christian (I'm not, so don't think I'm about to get preachy). He even goes so far as to use Latin as a parallel for an alignment language, as the religious languages often transcended the boundaries of national languages. Alignment language functions in much the same way with regard to the racial and national languages of AD&D. With Acolytes, the word is used to describe one who assists the Priests or Deacons in the performing of religious rites.
We can understand, then, what exactly our Cleric's "backstory" is by virtue of the title. The Cleric belongs to a widely worshipped deity, and served a church in the capacity of an Acolyte. This relationship with a Priest (level3 Cleric) becomes self-evident.
We can then begin to extrapolate some from the game mechanics. The Cleric is a more advanced martial fighter than the Thief and Magic-User, but less than all breeds of Fighter. He's not limited with respects to armor in any way, and is limited in weaponry so as to "not draw blood," drawing from stories of Odo of Bayeux in history. AD&D is more restrictive than some of the Basic iterations in that Clerics are forbidden from using Slings. The allowed weaponry are as follows: Club, Flail, Hammer, Mace, Staff. Some may cry foul that the Cleric's ranged potential is severely limited by the Sling's omission, but it should be remembered that Hammers are a thrown weapon type. Clerics are also expressly given permission to throw lit oil flasks. While Clerics may be tempted to rain holy fire upon their enemies to make up for the lack of a Sling, Gygax seemed to rein this in a bit in the DMG by saying that the flask would need to be prepared in advance for throwing, or it may not shatter on impact.
The advanced combat training hints toward a more militaristic existence than our historical parallels. The Clerics of D&D are ritualistic practitioners and capable warriors, which Gygax says "bears a resemblance to religious orders of knighthood in medieval times." We can infer from this that the detached, wandering Priest archetype would not fit what the material suggests, and instead that any PC Cleric is going to have strong ties to a massive religious organization throughout their careers.
Gygax's T1 (Village of Hommlet) supports this, at least to a degree. We have a Priest, a Canon, and then the former Canoness. Though the Canoness is off on a secret mission, the Canon and the Priest handle the duties for the Church of St. Cuthbert in Hommlet. We could infer that there is a duty of Clerics to grow the influence of their deities throughout the world (much like any religion) and that this entails local service at times, and adventuring to protect the flock at others. The PC Cleric adventurer may be a rare breed in that he is much more prone to questing than his Church-bound counterparts, or it may be that these things have a natural ebb and flow. At times, the questing must wait until there are enough Clerics to support an ongoing need of services, and at others, the dangers outside necessitate Clerics taking up arms and pushing back the tides of the enemy.
Looking more closely at the Cleric's starting abilities, we know that the Cleric knows how to invoke his deity in order to repel or command the undead, depending on alignment. Poul Anderson's Three Hearts and Three Lions gives a fantastic representation of the "Turn Unholy" in practice, which is fairly equivalent.
In the story, Holger knows that Elves and other unholy creatures cannot stand a true believer invoking the "Holy Names." When he does so, they retreat with great haste. He also knows that to do so multiple times in a row would be regarded as overstepping his bounds, so he restricts himself from employing the Holy Names too frequently. This mirrors the otherwise seemingly arbitrary limit that is placed on Clerics where they can only use Turn Undead once per creature type per encounter. This explanation presents a potential issue, however. Couldn't the Cleric just choose to risk his god's wrath? This is where the DM steps in. Perhaps he can risk a second attempt in times of peril, but there are increasingly higher chances of being refused spells until they have repented. Or, maybe the Turning functions much as it does for a Thief picking a lock, in that the undead he could not turn were simply beyond his skills.
The Level1 Cleric is able to cast spells, memorizing between 1 and 3 spells per day depending on the Cleric's Wisdom score. Since Wisdom reflects "enlightenment, judgment, wile, will power, and (to a certain extent) intuitiveness," we know that this is not a measure of the Cleric's mental capacity as it is for the Magic-User, but it's the level of enlightenment that the Cleric is capable of that determines how many spells per day can be cast. In this way, the leveling process for Clerics is a spiritual journey of sorts. The Clerics who sit idly, failing to accrue experience and train will find themselves forever behind those who go on great quests in the name of their deities and diligently improve their physical and spiritual status.
Casting spells requires memorization, but through prayer and recitation of holy writings. From the DMG, we get this tidbit: "...Clerics and Druids never need the aid of magic to read appropriate spell scrolls." (p118) The PHB tells us that Master Thieves can eventually decipher all scrolls except Clerical scrolls (p27). What this means for the Cleric is up for interpretation. My leaning is that there is some spark of the Divine in Clerics that makes them able to decipher these otherwise alien writings, and that it's not at all dependent on alignment or deity. A Cleric scroll is a Cleric scroll, after all. While the Master Thief is able to "fake it til you make it" with Arcane and Druidic scrolls, it seems doing so with holy/unholy writings is an insurmountable task. This gives Clerical scrolls questionably an even more Eldritch quality than the Magic-User's writings.
Due to the nature of how Clerics obtain their spells and the relationship with higher level Clerics, I would say that low level Clerics will be familiar with all manners of spells that are used within their deity's worship. They would have intimate knowledge of low level Cleric spells (1-2), some knowledge of moderate level Cleric spells (3-5), and a rough idea of high level Cleric spells (6-7). To be more explicit, let's take a Lawful Good Cleric of St. Cuthbert. At first level, I would expect that this Cleric would be familiar with all 1st and 2nd level spells. He may be aware that some of these spells can be reversed, but he'd know that it is forbidden. He wouldn't know how Animate Dead works, very likely, but he'd know that's where many Undead come from. If he had history of working with a High Priest, he'd probably understand how Raise Dead works and what its limitations are, as the spell would be a crucial one to society (keeping important people alive is good for civilization, and good for the church's coffers). He may have heard stories of Gate, Earthquake, Aerial Servant, etc, but he would know next to nothing about how they function.
The level1 Cleric's place in an adventuring party, practically speaking, is multifaceted. Being a spiritual leader among everyday people, the Cleric is prone to serving as an intermediary between the party and aligned religious organizations, or between the faithful populace and the otherwise contemptuous sellswords the party would be comprised of. The hit dice, armor choice, and hitting potential of the Cleric makes him a perfectly suitable melee combatant, but only once he has resigned himself to the melee, as casting at that point is far too dangerous. For similar reasons, being a middle-of-combat healer is a difficult prospect.
In AD&D, you cannot move and cast a spell in the same round. Also, if you do cast a spell, you're subject to interruptions from enemy attacks. Seeing as Cure Light Wounds is a 5 segment (thirty seconds!) spell, it can be easily interrupted by the enemy and result in a lost spell. Because of this, if you're using AD&D's or even B/X's BTB movement and initiative rules, you'd likely have to sacrifice at least two rounds and a potential failed casting in order to heal an ally in battle. This makes CLW a massive gamble that should likely be reserved for the direst of circumstances. In many cases, if your ally happens to survive the blow that gets him to 0 HP, you're better off letting him bleed a while and trying to finish the combat in the mean time, or simply stabilizing the wounds.
The Cleric is also kind of a rubbish ranged combatant. In 1E, his only options for ranged combat are to throw hammers, rocks (but not with a sling!), or oil flasks. With Hammers, the ranges are 1"/2"/3", which is frankly quite pathetic. Rocks without a sling would likely have similar effective ranges. Oil flasks are limited to 3", and Gygax ruled that they must be properly prepared in order to be lit and shatter with the normal chances (15 Normal Blow Saving Throw). So, his options are all limited to very close range, and are either low damage or require a round of preparation and a little luck to pull off.
More likely, the low level Cleric is best served turning undead, blinding enemies (with Light cast upon the eyes) that are tougher than average, and then engaging them, OR using healing spells between battles to keep the Fighters and Clerics in fighting condition.
There are edge cases where other spells may be more appropriately used. Sanctuary can be an excellent means to protect the Cleric while he goes around healing wounded party members. Protection From Evil can provide an incredible amount of safety to a Fighter, for instance, who can lock down an evil enemy, or protect anyone from enchanted/conjured creatures for a time. However, if he's expecting to encounter the latter kinds of creatures as a Level1 Cleric, he may be better off staying home.
This kind of conventional wisdom would likely be passed from Cleric to Cleric. Since there isn't really such thing as a Cleric who doesn't gather experience in the field, all higher level Clerics would be privvy to tactics and strategies that incorporate their god's blessings. For that reason, I'd say that any expectation that a level1 Cleric act as though he doesn't know what he's doing would be groundless.
The Acolyte becomes an Adept, a Priest, a Curate, a Perfect, a Canon, a Lama, a Patriarch, and finally a High Priest. Though these titles do not all belong to the same religion, they represent milestones in one's spiritual journey, finally resulting in the peak of what humanity can achieve. By that point, the High Priest will be creating magical items for Clerics, maintaining a religious stronghold, collecting taxes and tithes, managing a small army of followers, being a political force for the Church, and taking on the greatest threats the realm has to offer.
With that, I believe this article is too long to continue! I hope that there was something of use to be gleaned, or that we can at least battle ferociously in the comments.