Hey Shanny,
First, congrats! This will be such a fun (stressful, but fun) time for you.
To answer your question, we started trying to book in November 2007 for 2009 wedding but had to wait until the new rates were released. I think we were finally able to book in March or April of 2008, which isn't so bad. I would suggest you just keep checking back and remember if you can't book a particular date neither can other brides -- which keeps your date safe for the time being! )
As for notifying your guests, we sent save the dates before we even booked our actual hotel. It just said something along the lines of please save the date, we're getting married in Mexico. This gave people plenty of time to save and plan for a big vacation.
Hope this helps, best of luck on your planning!
Katrina