How to Roleplay!!!

Follow the "skirt rule" when developing your character. Basically, your character sheet should be "Long enough to cover the essentials, short enough to be interesting." Don't spend too much time on every single little detail of your character, but at the same time, don't make them a skeleton.
Make your character believable. In the RP world, perfect characters, called Mary Sues or Gary Stus, have a notorious reputation and are extremely frowned upon. As a plus, if you make a character with both good and bad traits, you are much more likely to have a lot of fun with him or her and the other people you are roleplaying with.
Most Character Sheets require a history. Here, you need to explain, explain, explain. Include everything you can think of, don't skip things to save time. Include descriptions of parents, places where character grew up etc.

If you are on a good RP site, there should be absolutely no text talking! Use full sentences and good punctuation and grammar. Use quotation marks when your character is talking. Otherwise many people get annoyed and you are made to look silly.
Never post one-liners! This is usually just one sentence of what your character is doing. Posting dialogue is not an excuse for a one-liner. The people who you roleplay with may not like it at all. The occasional one-liner is fine (barely) but do not make it a habit to where everyone else has the brunt of the roleplay. You are there too, so make the attempt to write longer posts.
Use description! Don't just say "Jane walked into the room and sat down on a chair" Where is Jane? What does the room look like? How did she walk in? How did she sit down? How was she feeling? What was she thinking as she walked in? Where had she just been? You need to think of all these questions whenever you RP. Instead, you could say something like, "Jane briskly walked into the waiting room at the doctors office, wrinkling her nose at the sickeningly strong smell of antiseptic. She glanced at the medical posters on the walls, and the magazines by the hard backed chairs lining the walls. She went, and quickly sat down in one of the uncomfortable plastic chairs, folding her ankles properly." Now do you see what you're aiming for?

RPing may seem like hard work, but you can really get into it and have a lot of fun. Following these suggestions above will help you have a great time, and make others on your RP quickly appreciate you.
Don't be afraid of adding too many details. The more details, the better off the roleplay will be. But don't add too much detail about one thing unless it is the main subject.
No one liners. They do nothing to advance the story. Most people do not like them as they do nothing for the roleplay, unless your thread starter says that one liners is okay, always assume if it is not mentioned that one liners are not permitted. They are allowed if you are just talking of course.
There are 2 ways to RP: One is to talk in third person, like: "Jane mercilessly tackles Jim, pinning him to the ground." The second one is to talk in first person, like: *tackles Jim, pinning him to the ground* But when you are talking in first person it is crucial to use asterisks. (*). But if you roleplay on another website and when you * something it gets bold-faced, just add ( - )