Player Creation: The Basics
The most important thing about your character is that you'll be using either Standard Array or Point Buy for ability scores, your choice, and you'll be doing it inside DNDBeyond. Hit Points should also be set to automatically calculate in DND Beyond.
You'll be starting at level 1, so multiclassing won't be a question (yet). As you level up, I allow it, but be warned that the most effective way to totally screw up a character is to badly implement multiple classes :) if you want to do it, there's no problem with it, but it will require more of a time investment on your part.
As far as optional player content, anything not considered homebrew in DNDBeyond is allowed; so, anything in Xan's or Tasha's, the monster player races in Volo's, Critical Role official content, and the latest Unearthed Arcana that's implemented in DNDBeyond - all good. new books, great, and arguably even stuff that's official on DM's guild, excepting things that are too much management overhead. DNDBeyond is great at taking the burden of doing things right on your character sheet, and giving me a single 'official' source so I'm not constantly asking about your character build, stats, bonuses and abilities - but that does slightly limit the range of things we can implement.
Any official background is fine, or feel free to build your own using the rules in the PHB/DNDBeyond. Keep in mind Background isn't the same thing as backstory. D&D doesn't do a great job of separating tactical game elements from flavor text, so be advised that if you want to have the bonuses you get from, say, Sailor, but your character is actually an out of work schoolteacher who has never sailed, that's totally fine. Pick sailor to get those proficiencies, and when it comes up, just remember your PC used to be a teacher.
If you choose a background or a class that requires some kind of lore from the world - a Cleric's god, a Warlock's Patron, a Criminal's contacts - feel free to let me know and I can fill the blank in, or just leave it open-ended until it comes up in game. We're not doing a true West Marches game (we have a set time, we have a set DM, and we're not a large group), but some of those elements are definitely here. I don't have a single massive story that I want to tell through your characters; I have a homebrew world but so far at least, we're not touching it for this game. Rather than a total freeform, however, we've got some classic elements and what arises out of that, we'll turn into the canon of this world as we go forward. And the small details and emotional history of your PC will always be in your control. If you're a Tabaxi who loves to swim, maybe they all do; maybe you're unique. That can be up to you.
For that matter, I try to avoid having monsters use effects like Charm and Friends when I can, because while it might be cool for "the Story" when your goliath barbarian gets turned into a TPK machine by the mean vampire, it actually kind of sucks for the game and definitely sucks for you as a player. Don't worry, I can substitute out the player-hypnosis stuff with other spells, or use them only in very specific defensive context when an NPC is trying to save its own skin, rather than as an offensive tactic. And stupidity can always lead to a TPK; Bodies teach lessons, too. The reverse is not true - your characters can and should use charm, friends, disguise, illusions and whatever else they can to gain an advantage over their adversaries.
Ultimately the important thing with the character is that you can wear it comfortably at the table. I encourage you to play PCs who aren't duplicates of yourself, too. Explore what it's like to be someone else, and you might be surprised how fun it can be.
You'll be starting at level 1, so multiclassing won't be a question (yet). As you level up, I allow it, but be warned that the most effective way to totally screw up a character is to badly implement multiple classes :) if you want to do it, there's no problem with it, but it will require more of a time investment on your part.
As far as optional player content, anything not considered homebrew in DNDBeyond is allowed; so, anything in Xan's or Tasha's, the monster player races in Volo's, Critical Role official content, and the latest Unearthed Arcana that's implemented in DNDBeyond - all good. new books, great, and arguably even stuff that's official on DM's guild, excepting things that are too much management overhead. DNDBeyond is great at taking the burden of doing things right on your character sheet, and giving me a single 'official' source so I'm not constantly asking about your character build, stats, bonuses and abilities - but that does slightly limit the range of things we can implement.
Any official background is fine, or feel free to build your own using the rules in the PHB/DNDBeyond. Keep in mind Background isn't the same thing as backstory. D&D doesn't do a great job of separating tactical game elements from flavor text, so be advised that if you want to have the bonuses you get from, say, Sailor, but your character is actually an out of work schoolteacher who has never sailed, that's totally fine. Pick sailor to get those proficiencies, and when it comes up, just remember your PC used to be a teacher.
If you choose a background or a class that requires some kind of lore from the world - a Cleric's god, a Warlock's Patron, a Criminal's contacts - feel free to let me know and I can fill the blank in, or just leave it open-ended until it comes up in game. We're not doing a true West Marches game (we have a set time, we have a set DM, and we're not a large group), but some of those elements are definitely here. I don't have a single massive story that I want to tell through your characters; I have a homebrew world but so far at least, we're not touching it for this game. Rather than a total freeform, however, we've got some classic elements and what arises out of that, we'll turn into the canon of this world as we go forward. And the small details and emotional history of your PC will always be in your control. If you're a Tabaxi who loves to swim, maybe they all do; maybe you're unique. That can be up to you.
For that matter, I try to avoid having monsters use effects like Charm and Friends when I can, because while it might be cool for "the Story" when your goliath barbarian gets turned into a TPK machine by the mean vampire, it actually kind of sucks for the game and definitely sucks for you as a player. Don't worry, I can substitute out the player-hypnosis stuff with other spells, or use them only in very specific defensive context when an NPC is trying to save its own skin, rather than as an offensive tactic. And stupidity can always lead to a TPK; Bodies teach lessons, too. The reverse is not true - your characters can and should use charm, friends, disguise, illusions and whatever else they can to gain an advantage over their adversaries.
Ultimately the important thing with the character is that you can wear it comfortably at the table. I encourage you to play PCs who aren't duplicates of yourself, too. Explore what it's like to be someone else, and you might be surprised how fun it can be.