Try getting some books about the GED, some preparation guides and after you've studied, get someone to quiz you on various items that you've studied. Don't try to study for hours on end, take breaks of 5-15 minutes then go back to studying. If you feel that the information is no longer sinking in, then shut the books because anymore and you'll find you'll be taking steps backwards.

From what I've been told about it, the GED test is rather easy, possibly easier than the normal high school final exams.