To kick things off, I like to go straight to the casual RP Out-Of-Character (OOC) sub-forum, and look for threads with one to three pages. You can then quickly read up what the Game Master (GM) has in store for you, and prepare a Character Sheet (CS). Things generally flow from there, and you quickly get the hang of it, especially when you're already used to RP's! You can venture into all the forums, and certainly don't let the Advanced RP category scare you away if you're a solid writer! You'll quickly find your preferences.
You can also check out the Guide that is in the top of the Introduction forum. Hope you have as much fun as I do!