I don't think the problem is clear. You can create a game, choose a sheet at creation time (5e OGL is probably your best bet), and create a character within that game. If you are invited to a game that has Character Vault access, you can move your character into that game. However, many GMs prefer you to create the character within their game once you have been accepted: they may have specific character generation requirements. Every game is a little different in this respect. You might want to look into Pick Up Games. That sounds a bit closer to what you are expecting from Roll20. Here's a wiki page that might help you find a game.
