For the Newb
Just what in the heck IS roleplaying?
Roleplaying is when you are playing someone else's role. Sounds simple? It can be. You've just gotta know how to work it.
Do I pick an original character (OC) or a character from a story?
This all depends on your personal preferences. I'm not even going to get into how much more work one is rather than another, because in all honesty I've been down both roads and either is as much work as you put into it.
So, what's it like to RP a character from a story?
I can't stress this enough. You need to do some research on the character you're playing. If you don't get them exactly right physically and mentally, people will notice. Pay attention to mannerisms, thought processes, what they're knowledgeable about. Make sure you can at least fake their knowledge base.
If you want more freedom, make an OC. If you mix your own ideas into a character, most people will not like that. They like the character as he/she is.