The GMAT is a computerized test designed to provide questions that are tailored to the test-taker's skill level. To find your skill level, you must complete a few questions -- the first five of each section. Your answers to these questions will determine what the rest of your test will be like. Answer poorly on the first few questions and the test will give you easier questions thereafter. You will answer them quickly, but it will take more of these questions to reach a high score level than it would if you aced the first five and had difficult questions to muddle through thereafter.
All of the questions ultimately matter, not just the first five. The GMAT penalizes test-takers for unanswered questions, so it is better to guess and finish than to skip a question. So, pace yourself. Know how much time you can spend on the first five questions without jeopardizing the rest of the test. Do what it takes to answer every question.
The multiple-choice answers on the GMAT are comprised of correct answers but also common wrong answers. These serve to bait test-takers into seeing the wrong answer as the right one. To avoid these answers, check your math problems for inverted numbers, mistakes in signs and simple math errors. The answers that result from common mistakes in solving these math problems are often hidden within the multiple-choice answers on the GMAT.
Another pitfall for test-takers is problems that they simply cannot solve because they do not know the answer. For these, eliminate the most obvious wrong answers and then make an educated guess. Again, the GMAT scorers penalize incomplete tests; placing a wrong answer is better than not filling it in at all.