The GED is a test based on knowledge and skill. You'll be tested in Math, English, Science and History--each of these in the forms of separate tests. It is possible to pass one or some of them without passing all of them. If you fail one, you will have the opportunity to study and come back and take just that one subject until you have passed it. Failing one subject does NOT mean you have to take all the subjects over again.
It's a good idea to use a GED diagnostic tool and find out which areas you need to study. Personally, for my GED students, I recommend the Kaplan GED study guide available at any bookstore or on Amazon. (You want the one for the current year.) It has a set of diagnostic tests that will help you determine your weaknesses. Then, it will provide you with the lessons you need to study. That way, you don't waste your time studying things you have already mastered.
You can always snag a tutor for all of your GED studies, or just for the ones you need a little help with. I love tutoring for the GED! It is very rewarding to both me and the student, and (so far) all of my students have passed the first time through!
(Note: I am not an affiliate partner with Kaplan or Amazon. It is just the tool I use and recommend.)