Here, I'll field this until someone comes up with a better explanation. *clears throat* A-hem. Uhm..
Roleplaying. Role playing. The idea to keep in mind is you're speaking as if you are your character, rather than yourself controlling the character. The difference, however, isn't readily there.
You: "Can someone pull from that mob?"
Your character: "Can someone pull from that mob?"
How is this different? It's not. That's where your character's personality comes in.
Maybe they're impatient: "Pull one of those lunkheads over here so we can fight 'em already!"
Or cautious: "It looks like we'll have a better chance against these guys if we pull one up front first."
Aside from base personality types, it's good to have a quirk of some sorts. Maybe the character isn't from earth and is still getting used to the customs here, so simple things like the monorail amaze and confuse them. Whatever the quirk is, make sure it isn't something that'll be annoying to other people.
Next up is the bio. Whether it's a sentence or a short story, every good roleplaying character needs one! Briefly detail your character's past. How did they get their powers? What made them come to Paragon City/The Rogue Isles to become a hero/villain? Do they like/dislike any hero/villain groups in particular? It's just a little flavor for yourself and other people to go on. If there's some big super secret whoa thing about your character, though, don't stick it in the bio. Keep it a secret so people can discover it through RP.
Tips when you're RPing in groups? Well, it's bound to get crowded, so try to talk a bit less. Always always ask questions about other people's characters. They put a lot of effort into them, and nothing is more delightful than someone taking interest in their hard work. 
In smaller teams, though, is where CHARACTER DEVELOPMENT happens! Characters are better to talk person-to-person here, rather than about the objectives. It's basically the same as getting to know a person in real life, only, again, your character. :P
Roleplaying on the forums? If you can mash your fingers on a keyboard, you can roleplay on the forums. Of course, without the videogame as a medium, you have to be detailed about your descriptions- including your character's thoughts is just as important as including speech and actions. Style is important, but if you don't think you have a way with words, just type how you talk. It'll give it a natural feel, at least.
Think of it as collaboratively writing a story- and to this end, feel free to collaborate with the other people in the threads, through private messages- to plan out and choreograph, to some degree, what you want to happen. It makes it more fun for everyone- just, be sure not to overdo it. There'll be things you want to flow smoothly, yeah, but don't micromanage- spontaenity, however you spell it, is fun. Also, don't godmode. I'll leave the details of that up to someone else.
I'm too tired to go on. Can someone else expain it from here, and elaborate on points I've been unclear on? *holds out relay baton*
I've been looking for a way to express how i feel about this game, i went to virtue the other night on my new character and saw how everyone was roleplaying.. so i want to too! Can anyone help me to be a better roleplayer please? thanks for your replys!