Use the character template supplied at the top of this section as a guide for building your character. You may want to add a character history to it as well, but that is not key. My main character is never accompanied by than three sentences in his history.
After you've made your character, depending on the sort of RP you want to be in (obviously there is no point in making an LotR character if you're going to enter a POTC RP) you want to find a roleplay to join. I recommend one that has only just got started. It is hard to join an RP that is more than 5 pages long (7 tops) because you have to cut in mid story. You also want to pick an RP that sounds like it has a good story line, with a leader who knows where they want to take it. basically, in the opening post there ought to be some form of long term objective, as well as an opening to get things going.
Other than that, the RP you choose is down to personal preferance, and what appeals to you, and the style of RP you are after. Some are more literate than others.
Then, once you've picked an RP, you join. It is generally considered polite to request permission to join from the leader of the roleplay first. Essentially, ask to join, and post your charrie's (character) bio for him/her to look over to check it is the right sort of thing, and wait to be accepted. [I'm yet to see someone told they can't join, so asking to join is just manners really

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Once you've been accepted you make a post introducing your character to the story so you can start interacting with other characters, and get involved with the plot. After that, just check back periodically and respond to what the other characters do. It is also considered polite and correct to let the leader (the RP starter) initiate the plot moving along, after all, it is their idea and they know best where to take it. [I personally have no problem with people advancing things in the same direction they are going in my RP's, but someone changing the direction would annoy me (some people like to be the only ones who move it along in any way at all)

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I hope that helps you some, and hasn't put you off the idea.

Happy roleplaying.
[And I hope it made sense too]