There are all sorts of ways to roleplay, but I personally have always thought of it as collaborative story-telling. Some people are very casual and just like to do things one line at a time, reacting to each other. Others are far more detailed (also called "literate") and put a lot of focus into their story-telling elements. They create entire worlds with their own natural laws, cultures, ethic codes and so on for their characters to live in. Usually, it's preferred to write in third person (Suzy walked to the candy story to buy some Mars Bars), and this should be a default unless otherwise stated.

But regardless of whether or not you write 1 line to 6 paragraphs+ per post, you ALWAYS need to give something for your RP partners to react to. It's very similar to acting/improv games in that way. Your characters can't accomplish anything and live their stories if the only thing you do with them is sit them under a tree. There will be instances where you might be left with some passive moments, but there should always be some opening for another player to interact with you. And nothing irritates other players more than when your character has been given some sort of responsibility and you don't do anything with it, dragging the story on until it fizzles and dies.

To me, that is the most important aspect to Roleplay. But the general "golden rule" is not to godmode your character. Your character should not be invincible. Should not be perfect in all ways (Also called a "Mary Sue"), because PEOPLE are not invincible and they have core flaws that make them the people that they are.

I've never done Table-Top gaming, so I'm not sure how any online RP translates. Though truthfully, I've always been curious about it. I just don't wanna play with all my friends 'cause they're major hardcore gamers.