To sign up, first read the rules and then go to sign up page to fill the sign up form (UXRL site > Drivers > Sign up).
EDIT: You also need to do a hotlap for pre-qualifying. More info: http://www.lfsforum.net/showthread.php?t=14351
EDIT: You also need to do a hotlap for pre-qualifying. More info: http://www.lfsforum.net/showthread.php?t=14351