Visit one of the GED study websites listed in the "Resources" section. The links down there will take you directly to their math pages. Some, like GEDSource, are more conceptual in nature; they offer you general tips about answering different types of questions. Others, like Study Guide Zone and TestPreview, offer quizzes. TestPreview actually offers self-assessment modules in ten different math-related subtopics.
Take the self-assessment quizzes or modules after you have reviewed the related concepts. Study Guide Zone and TestPreview offer refresher material for you to review before you try the problems, if you need to brush up.
Grade your answers against the keys provided. Study Guide Zone is an example of a site that explains why each correct answer is the one you should have chosen. Test Preview simply gives you a list of the correct answers.
Analyze the questions that you missed to find out which math skills are still lacking. Is it graphs? Fractions? Geometry? Take a list of those skills to a math tutor or to a review class, or read online tutorials in those areas, and then return to self-assessment modules to track your improvement.