I took them in '01, so I don’t remember them all that well. From what I do remember it is very similar to the SAT’s. I think that the math section is a little harder than the SAT’s, but the verbal is a little easier. The Analytical section is unique to the GMATs, but it wasn’t all that difficult.
My advice is just to take as many practice exams as you can, and maybe get a book from a name you trust (Kaplan, Princeton Review) and learn some strategies. Good luck!