If you hop into a RP thread, make sure your character doesn't use out-of-character knowledge.
Use some form of bracketting such as ((OOC: OOC stuff here.)) if you need to discuss things out-of-character with the other players in the RP thread you're in.
Introducing your character, you can always use this time to describe your character's appearance.
You don't always have to describe your character while RPing, unless someone new shows up, which it might be polite to describe your character again.
Any backstory for your character, you can use depending upon the situations while RPing.
There are other general rules about roleplaying, which other people here can elaborate upon in greater detail than I can.
step one is listen to Kyzock, the guy's good.
Step two, uh...
Be mindful of limitations. Having a character too powerful or with too convoluted a backstory (or too convoluted with too powerful a backstory...yes that can happen...how many kids does Satan have running around Paragon City now? Dude gets around) can be detrimental.
Also it helps if you think from a character's perspective, or 'write what you know'. For instance, I don't just use Japanese characters because I like anime (which I do, believe me I do LOL) I love the country and the culture, and try to use my knowledge to add spice to any given RP...at least as much as I can. The myths and legends, as well as cultural mannerisms, can add depth to a character. However, I would not make a Middle Eastern character, since I don't know enough about the region to justify using it as a backround, and any attempt would be two dimensional at best.
So, if you know a lot about a certain topic, try to incorporate it. Like, if you, hypothetically, know a lot about baseball, a baseball themed hero might be worthwhile. Silly, but worthwhile.
Homerun Hans! With the power to hit any ball! (Much more powerful than it sounds if you take enough liberties 
)
To take the limitations to the next level...
Build your character before you introduce them. Give them a strength. It can be powerful, as long as you give them an equally or more powerful weakness. Characters that struggle are more interesting than characters that don't.
Read the thread you want to get into. Read as far back as you are willing to understand what you are getting in to. The people will appreciate it.
Do not say you hit the person you attack unless it's your character as well. It's slower on action, but it is the only polite way to write. Also, don't dodge all the attacks against you. It's annoying to other people.
Show range of emotion. Don't have your characters always mad or sad or happy. It gets annoying no matter what emotion it is.
Use spell check. Make sure your grammar and spelling are good. Errors distract from your story.
I almost always write one huge goal for each character. It is what drives their story forward. Once they reach it, if they reach it, their story is over. Stop writing them. That's my opinion on that.
I want to try roleplaying, but need some guidance first. Anything that I should know?