Each campaign has its own setup; unfortunately there's no way to make one character (within Roll20) and carry it between campaigns. What you do, generally speaking, is use an online system such as Myth-Weavers and post your character information there; there are plenty of similar sites, but this one is pretty common. If the character creater you are already using allows others to view the character sheet you have produced, that works just fine as well. Then, when you join a campaign, the GM will assign you a token which you can fill out with basic information, generally hit points and one or two other basic statistics (armor class, movement, etc.) For more detailed character sheets, the GM may give you a Journal entry. This has to be designed by the GM for the specific campaign, and can be as simple or as detailed as s/he/it wishes it to be. I've seen entries that have a bare minimum, not much more than can be held by a token, and I've seen entries that include everything and anything the GM thinks might be useful. In my campaigns, I set up a system that includes character statistics, including bonuses, as well as hit points, armor class, base attacks, skills, etc etc. Then I set up a set of macros that reference those statistics, so that the player needs only to click a single button and the calculations are all done for them for any (most) given action their character might take... for example, a basic melee weapon references the character's weapon type, attack bonuses, damage bonuses, and other variables, adds the randomizer (d20) and outputs a nice clean sentence saying what was done, and the result of the roll. Unfortunately, there is no way for a player to carry this Journal entry between campaigns, at least not at the moment. GMs who purchase a paid membership do have a method of transferring these between campaigns of their own making, but that's not much help to players. Ask the GM of your game what his/her/its preferred method is, and follow their lead.
