Find a local continuing education center for adults and enroll in their General Equivalency Diploma classes. These classes are specially designed to help adults who had to drop out of high school catch up on all the academic areas that they missed. Take notes in class, even if you feel that the information is simple and that you will remember it. The test is stressful so do not be overconfident.
Use a General Equivalency Diploma study book and use it everyday. Make a schedule for yourself, say an hour every night after work, where you will sit down and work on a section. Go through all the sections in the study book and do not move to another section until you have completed the last section.
Take a practice General Equivalency Diploma test. The center where you are taking classes will offer these tests several times each year so take advantage of this. Taking a practice test will show you how you need to pace yourself and get rid of some of your test anxiety.
Take off a few days before the General Equivalency Diploma test and cram. go over your notes and study book. The issues covered are the same every time, math, science, history and english. The wording of the questions may change each year but the level of difficulty with stay the same.