What is your role?
What do I mean by role? Your role, in short, is where the RP starts, and you as a player stop.

Your role is not:

• Your class. A class is just a set of guidelines your character follows.
  
• Your appearance. Looks are just a description of your character, not a definition.
  
• Your character. Your character is just that, a character. In the over-arcing story of whatever RP you're in, your character is just a small piece of the larger whole.
  

Your role is:

• The story your character's life tells.
  
• Your character's place in the world, and it's relation to that world and her peoples.
  

Seems like I'm splitting hairs doesn't it? I'm not. One of the biggest factors to a good RP and good RP etiquette is knowing what you are doing. The most common mistake with RPing is forgetting that you're supposed to be telling your story and falling back on just character, class, and looks. For a RP to really shine, you need to add your story, not just your Mary Sue character description. The world you're in is a husk, an empty treasure chest, start typing up some gold.  

This tutorial is a great help. Thanks!