A computer-adaptive test is -- as the title says -- adaptive. That means that each time you answer a question, the computer adjusts to your responses when determining which question to present next. For example, the first question will be of moderate difficulty. If you answer it correctly, the next question will be more difficult. If you answer it incorrectly, the next question will be easier. The computer will continue presenting questions based on your responses, with the goal of determining your ability level.
It is very important to understand that questions at the beginning of a section affect your score more than those at the end. That's because the early questions are used to determine your general ability level. Once the computer determines your general ability level, it presents questions to identify your specific ability level. As you progress farther into a section, it will be difficult to raise your score very much, even if you answer most items correctly. That's because the later questions affect your score less, as they are used to pinpoint your exact score once the computer has identified your general ability level. Therefore, take as much time as you can afford to answer the early questions correctly. Your score on each section is based on the number of questions you answer correctly, as well as the difficulty level of those questions.

You need only minimal computer skills to take the computer-based GMAT. You will have plenty of time at the test center to work through a tutorial that allows you to practice such activities as answering questions, using the mouse, using the word processor (which you will need for your essay responses), and accessing the help function.