Studying using up to date GED specific material is key to doing well on the test. That means purchasing a GED study book like the Kaplan one mentioned above or I also really like the Princeton Study books. For someone who is particularly visual GED for Dummies can be really helpful but I don't think it covers everything as fully as those others.
Taking a practice test and then studying what you don't yet know is a good way to get started. I encourage my students to focus on preparing for one subject at a time. Each subject covers so much material that preparing for all of them at once can be overwhelming. My only exception to that is Math which I usually have people get started on in conjunction with one of the other topics. This is because there is just so much material to learn in math and it seems to "stick" better if it is learned a bit at a time.
Each subject needs to be approached differently.
For the Social Studies test you need a good overview of world and American history and the GED study books don't really give you that. I have other materials that I suggest to get students up to speed.
For Science you have a lot of terms to learn that may not be familiar. Also there is some math that needs to be known before you can pass the test. Being able to use a formula to solve some of the problems is key.
For Language Arts reading and understanding literature is key. The more you read and answer questions about what you read the better off in that subject. The essay can also be a big challenge for many students and it is important to get feedback from someone else on your essay writing skills since evaluating your own writing is very difficult to do.
Part of what I provide as a tutor is a plan to help my students stay on track and work toward their goals. I work together with a student to determine what level of outside work will fit into their life. I then provide homework and materials to use in that time so they can practice skills they learn in sessions and progress toward their goals as quickly as possible. Some of my students have a plan in place already and I support that but many have felt overwhelmed by the task and I am happy to break it down into easy to manage pieces.
The only thing I don't like about tutoring GED students is that they reach their goals so quickly that I am always saying goodbye to them.