You'll probably want to start off with an existing sheet. Pick one for a system you understand; knowing how it's supposed to work will help you figure out how it does work. Google probably knows about some HTML tutorials to help you figure out generally what's going on, and the Roll20 docs' section on character sheet creation should help fill in what's left. If that's too much to learn in time for your game, you can always just use the built-in character system (at least for now). Create a template character, set up attributes for the important stuff (forces, characteristics, hits, maybe a few important skills) as attributes, then create new characters by copying the template character. If I get a chance, I might try to throw something together later. However, I'm not very good at making visually appealing HTML pages, so it'd be pretty barebones (and untested, given that I'm not a subscriber).
