Creating a Character (new to Roll20)

I am new to this site but really only have one question. do I make a character before I join a game or will I make one in game if im accepted?

Depends on the game, the system, and the GM. I suggest finding a game and then finding out what the GM wants.

I've found a bunch that want a back story before they invite anyone. Having a back story kind of means you need to know what race/class you want to make. The only bad thing about this for me is that I dislike playing a class that is already in the game. Two fighters doing the same thing is kind of boring... plus it's frustrating if the other fighter min/maxes and is able to do... say 2D4+1D6+13 while you only do 2D4+6. It also gives me a chance to play different classes.

That depends on the GM like Gauss mentionend. It is allways a good idea of course, talking to the group before the session starts. Often a specific class is missing in the group. And your earlier you know what you are playing, you earlier can figure out how you are playing. Some thoughts about your char are always helpful playing the char. It makes a lot easier in the game.

the only problem I have would probably be that I hate playing common races (like the humanoid handbook races for example, Im a d&d guy) but most likely I will be stuck playing one of those

Tell this the GM and everything will be fine. There is always a way to handle special wishes from players.

k thank you guys