If you'd like to buy a GRE book, I can't say enough good things about Kaplan's GRE Study Guide.
You can take a diagnostic test to see where you really need to concentrate, and they use the Internet a lot to help with lessons. I was very happy with this book and my score.
If you aren't discipled (a lot of my friends weren't-they opened up a study book at home all the sudden they'd see a rerun of Full House was on and they had
to watch it, or at the library they'd decide to look up art history because that's near the section they were sitting in), I would recommend a class. I had a friend who did this because he wasn't disciplined and his parents were about to disown him if he didn't get into grad school, so he took prep courses. His score was really good, and the classes are supposed to be wonderful. However--of course there is a however--they are very expensive and time consuming (and you don't want to miss a course considering how much you are paying). The price for Kaplan's courses run $450 - $1200!!!
My last suggestion (especially if you don't have a lot of money) is checking out a book at the library and concentrating on the basic ideas you will be tested on. Instead of puchasing a book, you could go over the basic ideas from the free one from the library and take notes.
The main objectives to concentrate on:
1) Vocabulary. I made flash cards. You can also buy prepackaged GRE flashcards at any boookstore if you have extra money. Make sure you know word roots/prefixes.
2) Brush up on geometrey, algebra and data analysis. The flashcards I made I would take with me, so while waiting waiting for an appointment I would just start flipping though them.
The GRE isn't as scary as it sounds. A few weeks of just going over material you haven't seen in a while and you should good to go!!