Personality, Backstory, Goals, etc.
character personality
Generally I'll want to see around a paragraph (or more, if you've got it) about your character's personality. What's their usual demeanor? What sorts of people do they get along with? You don't have to go into excruciating detail here, nor do you have to worry about your character's backstory (more on that below). Just make sure that you know enough about who this person is and how they behave to be able to roleplay them alongside the rest of the party, and so that I have a decent understanding of what you're going for.
backstory and frontstory
I do not generally require detailed character backstories for my games, but I'm always more than happy to work with 'em if you've got 'em. Feel free to give me as much backstory info as you like, whether that be just a few sentences, a list of bullet points or a short bit of creative writing. (Do try not to write a whole novel, though. A few pages max is generally best. Save both of us some work and leave both of us some wiggle room for the future!) I will also gladly talk with you to brainstorm ideas or answer questions that will help you develop your backstory further. If you want to help me build the history of our world/the game, I'll weave your ideas into the narrative and make sure you and the other players can enjoy it, or possibly be stabbed in the back by it.
If, on the other hand, backstory isn't your thing, that's totally okay, too! I often find that I like jumping into a game without much backstory as well, so I totally get it. Backstory can be hard, or sometimes you're more interested in how your character will be affected by what's happening rather than what's already happened. As long as you have enough knowledge of your character's personality to have fun playing them and interacting with the party and the world, you're generally good to go. You can approach me at any time and say that you want to see more of something. "Hey, this renegade bugbear NPC is really cool; can you keep them involved in the story so we can interact more?" If a mid-game character or event interests you enough that you decide you want it to be a part of your character's history, I'm down for helping you develop a retroactive backstory that we can fit into the campaign as well.
Sometimes there may be small elements of backstory, history, connections or other information that I ask for if the structure of the campaign requires it. Any info threads for the game will say so if this is the case. If you're not normally a backstory person and you're struggling with this, just let me know and we can brainstorm, or maybe figure out something else that's more comfortable for your character-creating process.
character goals
In most games I'll also want an idea of your character's short-term and long-term goals. Individual games may ask for more info about this, such as specifying that your character should have a goal that is related to a specific thing. Feel free to talk to me privately about goals you don't want other players/characters to know right away.
This includes knowing what you as a player want for your character, even if your character doesn't know about this! Your character's goal might be to move to a new city; your goal for them might be to have them realize they're cold-hearted and then try to become more warm and caring over time. Maybe they're a devout follower of a deity, but you want something to happen that challenges their faith. Anything you can tell me about this sort of thing is also appreciated, but not necessarily required.
As with backstory, if you prefer to see how your character develops over time, that's okay—just let me know, or maybe provide a few loose suggestions if you're not sure yet. Some games may require a little bit of info to explain why the party is forming or something like that, but I'm okay with loose goals that may change as events warrant.
alignment
I don't make a huge deal out of alignment in my games. I do find it helpful as a jumping-off point for getting into your character's head, and there is still the odd vestigial mechanical effect that keys off of alignment, so I will usually ask for it; I'm not going to demand an alignment change or force you to switch your redemption paladin to an oathbreaker if you make a more morally-complex decision than "la la la obvious lawful good la la". If an alignment change does seem appropriate, you and I can discuss it at that time.
That said, I'm usually not a big fan of evil characters. This is not because I believe that "all evil characters are disruptive"—maybe it is a little easier to fall into that trap as an inexperienced player, but anyone who is actually considerate of the rest of their group can handle those alignments just fine—but more because evil just isn't my thing and I tend to prefer slightly more heroic parties and situations. Different games may have different alignment requests or restrictions, but the general rule is that I'd prefer you talk to me before creating an evil character, just to be sure that it's a good fit for the game and so that I can feel confident your character still has a reason to cooperate with the rest of the party.
player character connections
I'm totally okay with player characters who begin the game knowing one another. Just be sure you've cleared everything with the other player(s) in question first and let me know that you're doing it.