Have fun. If you don't enjoy creative writing, then this may not be your thing. You've gotta want to roleplay, not be forced into it.
Pace yourself. Not everyone can think up huge posts in minutes. Go at your own speed, and don't feel pressured to write a post just because others are leaving you behind in the dust. Ask them kindly to slow it down and wait for you. If they won't, maybe you should try and look for a different group of people to roleplay with.
Take breaks. Everyone gets burnt out and loses inspiration. Happens all the time. When you hit writers block, get up from your computer and do something else. Take a walk, go swimming, shoot some hoops. Sometimes inspiration hits you when you least expect it.
Be courteous. If someone is falling behind, don't send them fifty PMs reading "PSOT NAO!!!!!1!!!!11!!!" Wait a day or two, then send them one message asking them to kindly pick up the pace. If someone is new to roleplaying, don't insult them or talk down to them just because you might have more experience, or they made a few spelling or grammar mistakes. You probably started out this way yourself.
No flaming or trolling. Most people who have visited an online forum know what this means.
Be sure to use proper grammar and spelling, punctuation, etc. The only time when it'd be okay to misspell something would be when your character speaks with an accent of some kind. Consult a dictionary if you can't spell a word properly. If you find yourself using the same words over and over, the thesaurus is your friend.
And the biggest, most important rule of them all: No godmodding. Knowing things without having any possible way to know them, all these and more constitute godmodding. Mods don't take kindly to this sort of behavior, so it's best if you just avoid it.
Each person must start out as a patient. Then you may be a doctor. Only two characters allowed at max. Doctors and patients have different character skeletons.
