If possible, start preparing for the GRE test at least six months in advance. If you have less time, I recommend taking two different GRE prep courses at least a month apart so that you'll be better prepared. If you have the recommended amount of time to prepare, you may only need one course (though, if it helps you feel better prepared, by all means take more than one).
Research both GRE prep courses in your area and also those offered online. I used an online program and found it prepared me very well--my verbal score was in the 96th percentile (math was 94th). There weren't any conveniently scheduled local GRE prep courses for me, so that's why I selected an online program. Though I think the online option is a great one, if you can swing an in-person GRE prep class, I think you'd be wise to do so. You have to be more motivated to complete the program online.
If you have friends who've taken the GRE, find out how they prepared. This can help you find the right course based on your learning style. If you don't have any friends who've taken the test (or, at least not recently, like in my case), move on to the next step.
Kaplan and Princeton Review have two of the most well-known GRE prep courses, and both are very good programs. If you happen to have a private, local tutoring/preparatory service, definitely consider them as well. To decide between Kaplan and Princeton, take a look at both of their sites--they have different approaches to the test (I wound up using Princeton Review). Look for yourself, though, and see which one appeals to you based on your learning style and schedule/location needs.
Sign up for the GRE prep class. They'll let you know in advance if you'll need to bring anything or do anything else to prepare. I recommend running through at least one GRE prep book before your class so that you'll be familiar with the test's style and some of its unique features (the way it's scored is somewhat complicated and based on how you answered the previous questions). Good luck in both your GRE prep class and on the GRE test! I know you're going to do a great job :-).