Whether you are new to gaming, new to AD&D, or have been playing a while, you may see things on your sheet that aren't clear. This should help you understand what's on your sheet.
Quick disclaimer - I often modify the layout of sheets, so what you see may not be laid out exactly how your sheet looks, but it should be pretty close.
1. The Header - this section of your sheet has critical information about your character.
The sheet begins with your character name. The first line of the box will have your Dexterity, which I put there to make it easy for me to put attacks from the group in order - dexterity represents (among other things) your reaction time, so higher dexterity characters go first.
Residence - where you live.
Class - your character type (fighter, magic-user, etc.).
Race - the race you picked when you made your character - human, elf, etc.
Expenses - your monthly costs, in gold pieces. The default is 100gp per level; if you own property or a business, this can be higher or lower.
Alignment - whether you are good or evil (or in between, aka 'neutral'), whether you are lawful or chaotic (or in between). Sometimes written as an abbreviation - L/G for lawful/good, and so on.
Gender - whatever you chose at character creation.
Level - your character level, which gets higher as you play.
Gold Pieces - how much money you have.
Will Written - if you write a will for what you want to have happen if your character dies forever; a rare event in my game. I make it easy to get raised from the dead.
Maximum Age - this is a secret that only I know, the age at which your character will die of old age. Only important, usually, if your character is aged a lot in some magical way.
Hit points - the amount of damage you can take before being incapacitated.
Detect invisible - this is from a table that looks at your level and intelligence score.
Phone - so I can call you.
Character Improvement Points - each game, you will get 2-5 points you can accumulate and spend to improve your character. See this page for details.
Psionic Potential - another secret number that only I know, a very slight chance you will have psychic powers.
Email - so I can email you.
Treasure - often moved further down on the sheet, this is simply any 'loot' that isn't gold - gems, jewels, special metals, and the like.
Damage sustained - any damage you take, I record here; typically wiped away between games, unless the group ends a game in the middle of a battle or encounter.
2. Combat Chart
Your 'combat statistics' area.
Weapon - a special weapon you own, if magical, will have a comment like "+2" to the right.
+TH - shorthand for 'Plus To Hit', an adjustment to the 20 sided die you roll to attack.
+OD - shorthand for 'Plus On Damage', an adjustment to the damage die you roll on a hit.
Damage S/L - shorthand for 'Damage for Small or Mansized/Large creatures'. Often, but not always, weapons do more damage to large targets; above, you'll see that Conan's two-handed sword does 1-10 against small to man-sized targets, and 3-18 against large targets. ('Large' generally means creatures about the size of an ogre, maybe 8' tall and four hundred pounds in weight, or something comparable.) On the other hand, you'll note that Conan's long bow does 1-6 against any size creature.
#ATT - shorthand for 'number of attacks per round'. Combat in AD&D is described in 'melee rounds' or simply 'rounds', which are one minute long. Conan gets two attacks per round with his sword or his bow; this number can vary a lot, depending on weapon skills and weapon type. Sometimes this column will have a number like '5/2', which means '5 attacks every two rounds', so in round 1, you would get 3 attacks, and in round 2, you would only get 2, and so on.
Armor Class - one of the weird wargaming stats left over from the 1970s, Armor Class has a starting value of 10, indicating a person with no armor and no other traits that would help. Adding armor makes this number lower, and, confusingly, equipping a magic piece of armor with a description like 'plate mail +1' makes this number *lower*. Armor class is also reduced by very high dexterity scores, even though the bonus you get might be listed as something like 'Dex +1' on your sheet. To help keep these numbers straight, I will usually list above this line just what affects your armor class, like Conan has 'Chain Mail +4' and 'Dexterity +4'.
Saving Throw Adjustments - certain kinds of attacks, like magic or poison or dragon breath, can be mitigated or een avoided if you are lucky. You roll a 20 sided die, and hope for a high number. You may have an item (such as a 'ring of protection +1') that helps this roll, or armor (like Conan's 'chain mail +4'), or an ability (like Conan's very high Dexterity) that helps. Sometimes characters may have special abilities that make them more resistant to attacks; often you will see these on elves and dwarves, for instance.
Deity - this goes here for no particular reason, it's the deity you chose for your character.
Non-prof Penalty - a character can be 'proficient' with only a limited number of weapons; if you aren't proficient with a weapon, and try to use it, you get a reduction on the roll on the 20 sider you roll 'to hit', equal to this number.
Level of Next Proficiency - character gain new weapon proficiencies every so often; this number represents the level at which your character gains their next one.
3. Ratings Chart
Your 'ratings chart'. Each character has seven basic 'ratings' that describe their physical and mental characteristics, on a scale of 3-18 (though it's possible to go higher). The higher the number is, the better. If you want to get into the details of how these ratings affect play, look here; otherwise, what they do is pretty clear from the names - strength means you hit harder and can pick up more weight, and so on.
Enchantments - if you have gotten, somehow, a permanent spell on your person, it will show here.
Wish Level - you can wish to gain an experience level, but only one time (per class, if you have more than one class).
Book level - there are rare magical books that can add an experience level, but only one time (per class).
Aging - if you have been aged unnaturally, by monster attack or by magical effects, it is recorded here.
Haste Effect - probably the most common cause of unnatural aging is the level 3 magic-user spell Haste. This spell ages you one year - if you are a human. This aging is proportional to longer-lived races; elves, which live for hundreds or even a couple of thousand years, are aged much more by a Haste spell.
Drank Longevity Potions - how many of these you have consumed; they have the effect of removing magical aging. However, there is a 1% cumulative chance that each such potion you drink fails and wipes out all the years they have all removed, which can be deadly, so the number of such potions you have consumed is recorded.
Years Removed - as noted above, Longevity Potions can, if you drink enough of them, wipe out all the previous Longevity Potion effects, so the total number of years of unnatural aging removed is recorded. (Note, that like unnatural aging, this effect is proportionate by race.)
4. Magic Items
Magic Items - these sections will be either a chart of items (such as Charged Items), or just a list (like Magical Armor). If it's a chart, it's because the item has charges or uses (also because you are more likely to have a large number of potions, magical arrows, and the like).
5. Posessions and Character Background
Normal possessions and character background are listed here. You may accumulate a lot of 'stuff', and in general, if it's not listed here, you don't own it, so be sure to make a note that you own all the weapons you want to have for fighting, for instance. Below that, your background, much of which you pick; some of which I roll on a random table (the skill your family taught you, of which you know just a little generally); your weapon proficiencies, and your non-weapon proficiencies (there are dozens of these, and they can come in very handy).
6. Ability Score History and Hit Point History
Your ability scores are tracked, as they can change for many reasons.
Your hit points are also tracked, as they can also change sometimes. This shows how many hit points you gained at each level, adds in your constitution bonus if you have one, and shows the running 'grand total'.
I hope this has been helpful!