You would need to create your own campaign (free), using the character sheet for the system of your choice. Once you join a game, you can likely use the Character Vault (depending on the subscription level of the GM), to move it from your campaign to theirs. Note however, that many (if not most) GMs would prefer that you create the character within their campaign, so that they can guide the creation to meet the needs of the game, or any house rules or guidelines they employ. It will be different for every campaign. In any case, creating your own game will give you the practice and experience in character creation that will likely make the process more familiar.
