Well, first off this wouldn't be a post for the API forums.. That being said, you can build your character before you get accepted into a campaign so long as your DM approves doing so. Generally it's a good idea to get a sense of what the game will entail, the world will be like, and how many people will be in your party so you don't come up with anything to wild that will be difficult to play. As for the Roll20 character sheets, your DM will have to create one of those with all the applicable attributes and so on for you to fill out. You'll likely also have to fill out the bio, adding any notes you'd like to remember, as well as any macros that you'll need during gameplay. Hope that helps.
