First, you need to determine the amount of time you must devote to studying. This depends on your target score, application deadlines, and raw skills. We generally recommend that you study for about 3-6 months, but again, this really does depend on your personal situation. This blog post should be helpful in making your decision: http://magoosh.com/gre/2012/how-long-should-i-study-for-the-gre/
Next, you should determine your strengths and weaknesses. Are you great at math? Afraid of verbal? This will help guide your studies so you don't spend too much time reviewing concepts you're already good at, and accidentally skipping those that you struggle to master.
Once you've determined these two things (time length and weaknesses), you can choose from one our study schedules and set out on your way to conquering the GRE: http://magoosh.com/gre/gre-study-guides-and-plans/