There are free practice GED tests online.
Free GED Sample Test Online for Adults | GED®
Not sure if that link is truly free, but it was the first google hit. I'm sure there are free practice tests online somewhere if that link doesn't work.
I've always found practice tests to be a great place to start. It lets you know where your weaknesses are. It also lessens anxiety because you already have a feel for what the actual test will be like.