Ya may want to be more specific on this one. Are you asking 'how to roleplay' or 'how to roleplay on these boards'?
If it's the former, I got squat for ya. 
I've based all my characters on either myself, close friends, or people I otherwise know - and how they act, of course. Some of the more experienced people here who RP true fictional personas should be able to help fine, though. 
Tarnation, I just wrote an oxymoron. 
If it's the latter, there I can help ya out. Have some guidelines:
1) Be polite. Don't write the actions/reactions of other people's characters for them. There are exceptions to this, but hey, that's why it's called a guideline.
2) Characters exhibiting 'powers' tend to follow the way these work in the game. While some may not be quite accurate (Black Hole), slinging a lightning bolt usually doesn't need much modification. Again, this is only a guideline. Some characters have been shoehorned into the game and therefore tend to not follow those mechanics as precisely as others.
3) Try and keep with the spirit of things. If you enter a thread about...I dunno, gnomes or somethin'...dangit, I don't think I've been in this thing long enough to explain this. Basically, try to stick with what's in a thread when you enter it. Introducing something vastly different or more powerful can actually break a story very nicely. It's happened before.
Like I said, I may not be the person to go to here, but I hope I helped at least a little. 
"If I had Force powers, vacuum or not my cape/clothes/hair would always be blowing in the Dramatic Wind." - Tenzhi
Characters
Happy first post!
Hmmm, where to begin for roleplaying...
I'd start with a character idea. Develop it a little bit, get some story behind them, get the general feel for the character.
Basically, think of a person or thing (in some cases *cough*Khell*cough*), why they are where they are, what made them do this, where they came from. It'd be best to start out small, then add on details later on.
If you want you can send me a ingame tell at @Agent Darkstorm, I'll be glad to help you ingame.
--------------
For roleplaying on the fourms, basically everything DeviousMe has said. I don't recall if he added the ((brackets)) for OOC (Out of Character).
If you need RolePlaying help, feel free to chat ingame @Proto Zone...
Wait! That's it! I found the mission name for my global!
Potato Zone!!!!
In game, there's one thing I've seen that makes or breaks a character, and it's an important thing to have. From it, you can basically flush out every major detail of your character's life. I speak, of course, of speech patterns. Does your character have a southern drawl? Is his english impeccable, but with a slight accent, because he's from a foreign plane and learned it via some spell? Is he from the bronx, speaking with massive slang? (Careful with that one- some folks might simply get irritated at you for acting 'gangsta', as opposed to truly RPing.) My point is, figure out how your character speaks, and what they speak about. Give them a catchphrase or two. (In game, Harlequin Fear, a dark/dark stalker of mine uses the phrase 'I will consume' every time he enters a mission.) From there, you can flush out all sorts of minor details with simple improvisation, and your character is set. But, ya gotta get them speech patterns down. It's essential.
If you can avoid it, don't go 'OMG I'm having an emo day, brb.' then not RP post for about two months... <__< Not that I did that or anything, but still.
Nah, I'm kidding. Might want to get used to me before I insult you or something...
Yeah, anyway, everyone else is pretty much spot-on with their posts, and therefore, I really have nothing to add but annoyance and perhaps a little humor. ^__^
Oh, one thing that I find helpful lately is when you're typing a character's dialogue, try to speak with their voice inside your head. You'll end up choosing the ways you form your sentances differently for different types of characters.
Arc ID: 475246, "Bringing a Lord to Power"
"I'm only a simple man trying to cling to my tomorrow. Every day. By any means necessary."
-Caldwell B. Cladwell
I hope this will not be seen as an illegitimate post on this board.
I am interesting in RPing, but have no idea where or how to start. Boards such as these usually have a RolePlaying guide as a sticky, but I don't see a guide here.
Anyone care to write a guide or link me to one? I would really appreciate it.