Expect to spend about $25 on a good study guide. (I recommend the study guide by Kaplan available in bookstores or on Amazon. It will allow you to take diagnostic test to find out what areas you need to study.)
Depending on how much work you need, you can expect to spend 3-6 months preparing with only limited study time each week. (It can be done in less time with more dedicated study.)
Expect to take a series of four tests, one for each subject of Math, English, History, and Science. Once you have passed one subject, it is complete. If you fail a subject, you will have the opportunity to go back and take just that portion again. You will not have to retake any subject after you have passed it.
You may find that you can simply study on your own with a good study guide. Or, you might find that you need some tutoring in some of the more challenging areas. A good GED tutor will only need to tutor you on the areas where you need work.
(Note: I am not an affiliate for Kaplan or Amazon. It is simply the best tool I've found and the one I recommend.)