The Medical College Admission Test, or MCAT, is exactly like other standardized admission tests in that it creates a lot of stress for everyone taking it. It's also like other standardized admission tests in that you can prepare for it if you have to take it. It's normal to be nervous about the test, but don't let the pressure get to you. Not only can you take it multiple times, but you have a number of ways to prepare ahead of time.
Get your hands on every practice test you can find. Taking a few can help calm you down once you learn the format and find those subjects for which you need more review. Taking as many practice tests as possible helps even more because, when you're done with those, you'll have a good sense of how you've improved over time -- and you'll be so familiar with the format that when you take the real MCAT it's not going to be as mysterious and threatening to you.
If you can afford to, take some MCAT preparation courses that focus on strategies for taking the test as efficiently as possible. You'll get more practice tests, more practice problems, and a better idea of what you should focus on when studying. These courses have different ways of approaching the test, so if you have the opportunity to attend a free intro class, for example, do that. If you're going to spend money on these classes, you have to be sure the strategies the class uses actually seem helpful to you.
You also need to address your stress levels because those can make you freeze up on the test no matter how well you've prepared. You don't have to go very in-depth, and many meditation "courses" are available for free online. Look at medical centers and university research units; their websites often have free resources. Local community centers and your own university's recreation-class department should have additional low-cost seminars that you could take. You don't have to spend a lot of money, if any, for this preparation. The meditation will help you manage your stress and remove some of the chattering-mind phenomena that can trip you up.
Start now and pace yourself. Remind yourself you can take the test again and that everyone is feeling just as nervous as you. People get a range of scores on this test and still go on to do great things.