If you are applying to grad school and plan to take the GRE, get serious about test prep at least several months before your exam, and prepare to spend significant time in order to boost your scores. Get started with a good GRE prep book, such as Kaplan, The Official Guide to the GRE or the 5 lb Book of GRE Practice Problems. (Here's a page of popular GRE texts on Amazon. Check out the number of 4-5 starred reviews for a guide to how helpful people have found the books.)
Kick off in whatever section you can. If math is your biggest challenge, begin there. If you come to a topic that you find especially difficult, mark the page and go back to it with help later either with a teacher or do some Googling on that topic. These math websites are particularly helpful: purplemath.com, mathisfun.com, regentsprep.org. The textbook GRE for Dummies is very helpful on math basics.
Also, set out right now to build and strengthen your vocabulary. Register for a free account on quizlet.com and make your own flashcards, including definitions and defining sentences for each word. Practice your flashcards throughout the day on your cellphone so you learn at least 5-10 new words a day – and retain them! Get a Kindle or iPad if you can so that you can see the definitions of new words as you read, and do some reading in challenging English every day. Read The New Yorker or whatever well-written English language publication interests you. Make it a point to read at least one article in The New York Times, The Wall Street Journal, The Times of London, or other major newspaper. Knowledge of current events and the vocabulary of politics and business will help you not just in the vocabulary questions, but in the analytical essays on this test. If you don't have a eReader to quickly get new definitions as you read books, consider the Kaplan SAT vocabulary classics. These classic novels are presented with the hard words in bold and their definitions on the facing pages for quick learning withoug interrupting your reading flow.
If you can afford a good tutor, get one. Otherwise, try a class. What a good tutor or class will add to the mix is teaching you test-taking techniques, explaining areas that are challenging to you, guiding you to additional practice to build those skills which the teacher sees that need to be strengthened, and coaching you along in your progress, assigning you homework and keeping things moving forward at a strong pace.