The company that arranges the exam will give you a few sample questions in a booklet on request, and their web-site a few more, but try to get hold of a preparation book. I really think that practice is the key. Though the results of the test are not the only factor a university considers by any means, they are still carefully taken into account. Don't be too hard on yourself if you don't do as well as you had hoped but make an effort to prepare for it. Once you've been accepted and you're in an American classroom you will notice some differences.
Most graduate classes are in a way set up more like undergraduate classes at home.
You may have to select a number of different courses each term and not really feel the specialization of a British Masters. Also, you will find the required reading is often extensive and may seem more guided by the professor than in England. Finally, let me say a few things that you will probably find in every book about studying abroad but are not made clear until you are here. First, you will experience some kind of home sickness. Second, you probably will learn as much about Britain and the things you leave behind as you do about America and the things you arrive to.