@Sarzael: Well, RP dose not have many rules. It's mostly freeform, and yet...here's the ones I tend to follow. It's long winded but not very informative!
Stuff
1. Don't take liberties with others peoples chars. Pretty self explanatory anti-metagaming. "I hit him and he flies back!' vs 'I aim a punch at his head and swing!'. On the other hand, describing how someone might feel something differently could be allowed-like 'Tiruin can feel hostility boiling from Dwarmin." etc, etc
2. Don't do anything that makes anyone super uncomfortable without making sure they're okay with it-usually in PM, but not needed very often. I'm no stranger to romantic subplots, myself, but make sure you don't skeeve anyone out. Likewise, don't be a jerk to everyone. Even if your character is a jerk. Some people are okay with romance stuff, others can handle aggression and dish it back. Try to get a feel for how everyone wants to play.
3. Improvise alot but stay within the bounds of the games reality. Unless your psychic, you won't know another persons thought text or their background unless they tell it to you. Try to keep the game as real as it will allow, so as not to break immersion. Likewise, RP isnt a concrete thing that can't ever be violated-if you need to shoehorn in some information you might know so we can move forward in the game or perhaps not die horribly, don't be shy...
4. Be descriptive. Describe your chars thoughts and feelings-as with #3, we can't read your mind but we know the emotion on your 'face'. It helps everyone understand what sort of char you're playing. And helps flesh out the world. Which is a big help to a GM. If you're particularly bold, and feel you understand the world enough, you can inject stories into your RP-which, will hopefully be incorporated by the GM in some way. We're helping him build the world around us, ideally enough.
If you RP long enough, all this will start to come fairly naturally.