I'll explain the basics as well as I can, but much of roleplaying comes from observing and participating. First, let me point out, it's generally better to participate in, at the very least, one or two roleplays before you create one yourself. Doesn't have to have been here(If you came from another forum where you roleplayed, etc.), but it'll be clear whether or not you've been in one.
Let's start with the very basics. Role(a part or character)Playing(Should be obvious). Roleplaying is the art(or game) of taking the role of a character in a story written by and alongside other people doing the same. As a player, you start by creating a character. This character is your character, and you write out his/her actions in the world of the RP you joined. If you frequent the forum games, you may have already participated in an extremely casual version of an RP without knowing it.
The best way to learn how to roleplay is to join one. Observe the other roleplayers and how they structure their posts. Post as your own character. If the other roleplayers offer you advice, listen. And don't be afraid to ask for advice, either.
I hope I've provided enough for you to join a roleplay and start learning. If you need more advice, I would recommend either
PMing me. I'm an experienced roleplayer, and I've participated in a good deal of roleplays, both on the casual and experienced level. I'm willing to answer any questions about roleplaying you might have.