Easiest way to do a serious RP is to join one, check the serious and fun RP boards for one that looks interesting, and sign up. ((Makes sure you sign up in the serious companion board for the serious RPs, 90% of serious RPs want to keep the OOC stuff separate))
Bigger question is what kind of RP do you want to be part of? we have quite a variety right now with the crushingly dark EverE Factory, and one that is literally just a high-school.
Be prepared that different RPs have different paces, the Fun RPs you can have issues with if you are not on at the relevant times, while serious RPs generally take place over months, with maybe 2 posts per person if your lucky
Unless you have any specific questions I can't really say much more that isn't already in the stickied threads...
