Alright, basically, AAAAXEEEMAAAAAAN, how this all works is that there are 'threads', which have 3 little parts in them (In Character, Out of Character, and Characters), and you do stuff. Look around using one of my threads (for example) to get an idea:
http://www.roleplayerguild.com/topics/82487-plague-hunters/oocthe "In Character" thing is meant for the actual RP itself, the "Out of character" is meant for discussions, and the "Characters" tab is basically a cache of "character sheets" so everyone can just click on it, then can get an idea on what each character is (assuming they have a relevant post).
Also, if you want to use one character for many RP's, May I suggest creating your own thread under the "Character Sheets' sub forums? So you can just make your sheet, post it, then link it to a GM for approval. However, if you do do that, It's still commonly preferred to copy it over to that RP threads' character tab anyway.
Hit me up if you need anything else, I've been here for a month or two so I know some stuff. Ask a moderator like "Heyseuss" (the one I usually go to to ask my stupid questions), or "Mahz" (But he's our grumpy admin, he doesn't like people wasting his time. Only tell him super-important stuff like 'ermagerd the site is exploding')