It's pretty easy! Just hop into any interest check board and scroll through the currently recruiting RPs, I'm sure you'll find something to your liking. A good place to start if you're super new is Free, but Casual is also pretty accepting of new players. Most 1x1 people will also be willing to get you up to speed on things.

The best thing to do is just look for something and give it a shot. Each RP has their own CS that they'll give to you, so it's just a matter of filling that in. If you're confused, just look at what others do and try to imitate it (but with your own original ideas, of course!)

Hope you have a nice stay!

Also, if you're into ship girls, I'm currently eyeing an RP myself that doesn't seem to look too hard for a newcomer. I'll PM you a link if you have any interest in the idea.