Oh, by all means write your story out of the game in Text Edit (or the like) but always keep your character evolving. I find the best roleplay comes while playing, then I write down what I said abou my character In Game so I can keep his past consistent.
You can edit your character description though, which is a brief overview of what your character looks like, or what your character has done. Open up the Skills menu from the buttons while playing, click to the right of the eye icon (the icon for editing descriptions is missing) Then delete the text in the box that appears and write your own. After that, click the bottom =->LEFT<-= of the text box (the button to save is also missing). Try to keep what you put in there reasonable and in character.
Hope to see you around!
EDIT: Note the big giant left O.o
