Get your hands on a practice test -- one of the real exams, preferably. Take it under timed conditions,and then evaluate your score. If nothing else, it will help you identify areas of weakness and give you a chance to work on those areas, specifically. It will also give you a sense of how much time you should take to prepare for the GRE. If you start out with great scores, you won't need to take much time to study.
When I was preparing for the GRE, my scores on the practice exam I took to get a baseline were something like: >90th percentile in both analytical and verbal (I majored in history, so reading and critical thinking were skills I'd been practicing for some years.), aaaaaand bringing up the rear, we had: <30th percentile in math.
Suffice to say, math is NOT my strong suit. I used a book called, 'math for standardized tests' to work on my math skills, and in the real exam I scored somewhere around the 60th percentile in math. (It might not sound like much, but considering where I'd started, it was a huge victory for me.)
Math for Standardized Tests was nicely targeted. i could pick and choose the chapters I wanted to study based on whether or not the material would be on the exam I was taking.
Good luck!