Also, check to see if there is a planning thread. If there is, ask in there what the RP is about, ask who the GM (Game Master) is and what the rules are for the RP. Is there a story line? What is the basic thought of the game? What time period is it set in? How many players are allowed in?
Then, you need to come up with a character.
For a character you need:
Then an optional thing is a back history. Some people like to be vague on these and make people keep guessing. Some people like to be detailed so everyone knows what's up immediately. It takes all sorts for an interesting RP.
Anyway, do you have any other questions?