The mods explained the sign up steps over here inyuletide_admin, but feel free to IM or email me if you have any questions or are unsure about anything. I was a noob last year, so I'm up for hand-holding anyone who needs guidance this time around. ^_^
Check out the List of Nominated Fandoms to see what you have to choose from for both your three or four story requests and to narrow down a list of which fandoms you can offer to write. Don't be put off from requesting a fandom just because no one's offered it or vice versa; people will be signing up and making changes right up to the deadline. If you're not sure whether or not you're comfortable writing for all the characters in a given fandom, you can take a look at the Master Fandom Character List to be sure.
So does anyone have any questions? Hit me with 'em if so!