Hello and welcome!
You can book either way. First of all, don't fret -- the folks at Member Services are great, and will help you with the reservation process. You can call at the 11-month anniversary of checkout or you can call each morning, or do a "somewhere in between" and book a few nights at a time. Your decision will be based on availability for the time of year you are traveling (is it Christmas week?) as well as your personality. I am a hysterical type and I always call day-by-day. If you call daily, let them know you'll be calling back the next day or so. When you ultimately complete the reservation, they'll send out your confirmation letter.
Calling daily is probably more appropriate when booking at somehwere besides your home resort. By the 7-month window, lots of rooms are already theoretically "gone" so calling each day is more important. I called daily for a few days at Hilton Head, and had no problems getting a room. We just got back and we loved it!
Again, welcome and good luck.