Assuming you are in college, I would not recommend preparing too early because the GRE is a fairly general exam consisting of reading, writing, and math. Instead, I would focus on learning as much as you can in your coursework, reading everything that is assigned because this will improve your reading comprehension and vocabulary. I also recommend using the college's writing center to improve your writing and grammar. As for math, take math and use your school's math lab or tutoring services to fully understand the concepts. The key is you are learning for the sake of learning the material long term, not just to pass the class. Having studied a foreign language, always help because it improves your ability to understand unknown words in context.
If you're not currently in school, begin by reading college-level works. The New York Times Bestseller List may be a good place to start. Consider making a vocabulary list on Vocabulary.com of the words you do not know as you're reading. Create a Khan Academy account to work on your math skills at khanacademy.com. You could also create an account on March2Success.com, a site offered by the US Army to help students prepare for standardized exams. If you do not want to be contacted by a recruiter, just check the box stating "Do not contact me." This will help you review questions like those that will appear on the test. FYI: these three sites are all free to use without restrictions.
About three to six months before the test is when I'd begin to start getting comfortable with the test. (Honestly, I procrastinated with this part.) The big thing is that at this point you need to understand how the test works so you are essentially "learning the rules to the game" so to speak. The GRE is not something for which you can easily cram. Rather it's a test that attempts to measure your verbal reasoning and reading comprehension skills, in addition to your mathematical and logical reasoning skills. This all occurs within the the multiple choice sections. The writing section tests your ability to write a coherent and logical argument from beginning to end. (Of course, while the GRE creators assert that the GRE does all these things, I leave it up to you to evaluate to what extent the test measures what it claims to measure.)
Please let me know how I may further assist you!