At worse, you may need to brush up on basic algebra and geometry, reading and writing skills. When I took the test, I did it at SJSU on paper. Also, when I took it, The school scheduled the GMAT only four times a year. Now, with the computer-based system, it can be taken nearly at will. Time constraints are used to create this pressure.
Preparation includes strategy.
Basically, what you need to do is develop a strategy for taking the test. Purchase GMAT test preparation software. Another alternative is to purchase a test prep book, but if you are taking the test on computer, it's best to practice on a computer. There are several on the market. I used Kaplan's.