Creating a character can be a daunting process, especially in 1st edition AD&D, and doubly so if you don't have the books as a reference. I will outline the basics here to help you at least begin to understand what you need to know. If you don't have your own copy of the Player's Handbook, The OSRIC rules are close enough to get you started. (Warning, very large PDF at that link). You should also read the house rules.
For the most part, if you say you know what you are doing and have read the house rules, I will trust you. If you're not sure, ask. If you've only played other editions, ask someone who knows what the differences are. If you don't have a copy of the 1st edition rules, use the OSRIC ones linked above rather than assuming they are the same rules you are familiar with, or borrow books during the game session. It's been over 30 years since 1st Edition AD&D was first published. The rules have changed a lot over that time.
Very few online tools or applications support 1st edition rules!
See the Frequently Asked Questions page for examples. The longer ones are particularly helpful at providing spaces and blanks that help you fill out not only the bare bones information but the details, quirks, and helpful reference information needed to make the game flow smoothly.
Humans are the default. Other allowed races include elves, dwarves, halflings, gnomes, half-elves, and half-orcs. Subraces from Unearthed Arcana are allowed. Underdark races (dark elves/drow, deep gnomes/svirfneblin, grey dwarves/duergar) are not. Weird races from later editions (planetouched, etc) are not, although someone with a really really interesting character idea might get me to allow an aasimar or tiefling. Might.
Roll 4d6. Remove the lowest and add the remaining three dice together. Write down the result. Repeat 6 times. Assign those values to your Strength, Intelligence, Wisdom, Dexterity, Constitution, and Charisma. (If you are using old school rules, assign them in order; newbies can assign however they like). Look up the Comeliness modifiers in Unearthed Arcana for your Charisma and race and roll that, too.
Your attributes may give you bonuses or penalties. Your character sheet may have space for those. Look them up and fill those spaces in (newbies, do so after you pick a class!)
If you're not sure whether you are playing by newbie or old-school rules, see the house rules on that topic. Basically, it's up to you, but old school rules emphasize rolling the dice and making the best (and/or most interesting) of the result, while newbie rules emphasize being able to decide what sort of character you want to play ahead of time. Old school rules will also present a greater challenge.
Your options are: Fighter, Paladin, Ranger, Barbarian, Thief, Assassin, Cleric, Druid, Magic-User, Illusionist, Monk, Cavalier, and possibly a few others I've forgotten. Non-human races can multiclass (progress in two or more classes simultaneously). Humans can dual-class (switch classes after the game has started).
You must meet the minimum attribute requirements of the class you chose. Newbies may move points between attributes to meet the minimum requirements of their desired class. Your race and class must also be compatible (some classes only allow some races). Your choice of class may also dictate some other choices (paladins must be of lawful good alignment, druids must be of true neutral alignment, monks must be of any lawful alignment, barbarians cannot be lawful alignment, and so on).
Choosing a class will often come with important other choices. Magic users may need to determine the contents of their spellbook, clerics and paladins should consider the nature of their character's faith (but need not actually pick a deity until later in the game), thieves and assassins should consider whether they are open about their profession (which may not be popular) if it they try to hide it, and so on.
Roll a die of the type specified by your character class and add your Constitution bonus, if any. If your roll is low, check the Unearthed Arcana starting hit point mercy rules, towards the back.
ALmost all characters will speak common (an international trade pidgin of sorts suitable for simple communication), the language of their national origin if it has one, and their racial and alignment tongues. Your intelligence may give you the ability to speak additional languages. Choose them, if you wish, or learn them during gameplay. Humans do not have a racial tongue; their national tongue takes its place. Racial crossbreeds (half-orcs, half-elves) speak the racial tongue of their non-human half, usually with a noticeable accent.
Rookroost has no national tongue. Most communications happen in common, which is generally adequate for threats, bribes, coercion, and taxation. There are enough half-orcs in government positions that their language is available to anyone who wants to take the trouble to learn. Those starting the game in Dunwitch may have picked up elven or dwarven as well.
You get a number of these to start with determined by your class. These are basically a specific type of weapon; not "sword", but "long sword", "short sword", "broad sword", "two handed sword", and so on. If you are using a weapon you are not proficient with, you will take a penalty to your to-hit rolls. The exact penalty varies by class. Fighters should check the specialization rules in Unearthed Arcana. Check your class to make sure you are allowed to use the weapons you take proficiencies in.
See the house rules on this topic.
Starting gold is determined by your class and a random die roll. Equipment lists and prices are found in the player's handbook for most common adventuring needs.
Make sure you purchase weapons you are proficient at using. Calculate your armor class based on the armor you buy. Make sure your class is actually allowed to wear that armor type. Make sure your strength is sufficient to carry the equipment and armor you buy. (Unless you have a very low strength and heavy armor, you're probably going to be ok, at least until you start trying to carry home a dungeon's worth of treasure).
Determine your base THAC0 and your THAC0 for each primary weapon. Write it down. Write down the damage done by each primary weapon as well. This is so you don't have to look it up later. Your THAC0 and damage can be affected by your class, your strength, the weapon type (and any enchantments), the size class of your opponent, and whether you specialized or double specialized in that weapon type.
Thieves have an extensive set of skills affected by dexterity score, race, and type of armor worn. Many classes have spellcasting abilities. Paladins have religious abilities. Monks have weird abilities. Many races also have special abilities like finding secret doors, seeing in darkness, immunity or resistance to certain spells, and so on.
As a player, it is your responsibility to know your character's abilities. The Dungeon Master will be able to rule on what you can do, if you bring it up, but you have to be prepared to bring it up. The other players can help you figure out the details if the Dungeon Master is busy with someone else. If you don't know, ask.
Magic-users and illusionists must determine the contents of their spellbook. You will receive a few starting spells (ask the DM; the rules are in the DMG), and you can find scrolls or spellbooks as you adventure to gain more. When you gain access to a new level of spells (spell level, NOT character level), you will receive one spell of the new level automatically from the person who is training you. You may be able to request a specific spell, usually for a fee.
Divine spellcasters receive access to all of their classes' spells (by level), but must still choose which spells to memorize ahead of time.
Your intelligence and/or wisdom may offer you bonuses here.
Alignment is basically a moral outlook on life. There are two axes: law/chaos and good/evil, with a "neutral" option in the middle of them both. You pick one value from both axes. Thus a typical alignment could be "Chaotic Good" or "Neutral Evil" or "Lawful Neutral". "True Neutral" represents a desire to maintain a conscious balance between all extremes. Chaotic Neutral characters tend to take actions without any consistent moral or philosophical reasoning behind them. They can be played reasonably or stupidly. Those played stupidly may be struck by meteors at random. Some classes restrict your alignment; make sure yours is compatible.
Do I really need to explain this? Just make sure you have one. Or two, or even three.
Hint: Magi characters may wish to choose a "true name" and keep it separate and secret from the name they use every day...
What does your character look like to other people? Are they brash? Confident? Shy? Grim and menacing? Carefree? Nervous? How did they get to be that way? Why did they decide to seek adventure? Were they traumatized as a child and now seek to protect others? Did they decide they liked taking lunch money from other kids and wanted to make a living at it?
In addition to general stuff, it may be helpful to have an idea where in the Greyhawk world you were born and raised. The default will be the starting location of Dunwitch, but new characters can mostly pick whatever seems convenient for linking up with the party.
It may also be helpful (particularly for interactions with other party members, or if your character is a cleric or paladin) to know what your character's take on religion is. The world of Oerth offers many possible patron dieties. Characters can follow any of those generally available, or make something up, or nothing at all. Unless you are a cleric or paladin, it won't have a significant effect on actual gameplay.