Like the other posters have mentioned: (1) the MCAT is nothing like the GRE. Prepping for one will not help you on the other. (2) The GRE is very easy. If you're used to taking standardized tests, you should be fine.
The GRE consists of
- high school-level math (really! nothing hard at all)
- reading comprehension and definitions of words
- critical writing section
Overall, the test resembles the SAT (it's by the same company). You can sign up to take the test whenever you want, show up, take a computer-based exam in a cubicle all by yourself, and three hours later you go on with your life as if nothing happened. Since the exam is computer-based, you get your scores for verbal and math instantly---writing comes in the mail six weeks later. Any other questions: see the Kaplan book. It's the best of the series (TPR vs. Kaplan vs. Barron vs. etc...)