Sign up at www.mba.com
From there you can click "Register for GMAT" or whatever the button is called and it will take you to a screen where you can select test centers in your area and see appointment availability. I live in a large metro area and there are several test centers, and I've checked periodically and you can sign up really soon if you want. I checked a few days ago and saw that I could even take the test this week if I wanted.
The closer you are to the actual day you want to take the test, the less flexibility you will have in choosing specific days/times. I would say just to be safe, sign up at least 2-3 weeks in advance.