Be sure to have finished your pre-registration requirements before Oct. 1, if only to speed along the rest of the process.
FIRST has a Calendar of Important Deadlines on their website:
http://www.usfirst.org/community/frc...nt.aspx?id=454
That'll help you to remember when the fees are due for your first event and any subsequent events that you might participate.
You don't have to pay when you register for an event, but your spot at the event will not be assured until you have paid. Put another way, if a regional has capacity for 40 teams and you're the first to sign up, but the 41st to pay, you won't have a spot. This rarely happens, though, and as a rookie team your regional director ought to be looking out for you and making sure you have space to compete in your home event.