Enroll in a GED study program. Find one by searching the National Literacy Directory's website or calling 1-800-38YOURGED. Many of these programs are affiliated with local colleges. Most of these programs are free or very low cos. They provide instruction and practice tests.
Use a GED study book to learn what to expect on the test. You can find these books in most book stores. You also can find comprehensive study guides or a book focused on a particular portion of the exam.
Take practice tests in your books or online at one of the various GED practice websites such as Test Prep Review or Steck Vaughn's Adult Education. Determine areas where you need to improve, and continue studying them until you can consistently pass the practice tests.