Multiple choice tests are often thought of as the easiest kind of test to take, but that's not necessarily true. You're confronted with four or five possible answers to a question, and you have to figure out the right answer. That can be difficult, especially if there isn't one clear answer. Also, you may be psyching yourself out because you've taken these tests before and know there may be trick questions or that questions may be worded strangely, such as "Which one of these does NOT belong . . ."
When taking multiple choice tests, the first thing to do is to read each question very carefully (several times if necessary) to make sure it's not a trick question and to pick up on any key words that will help you answer the question, such as not. If you already know the answer to the question, look for that answer in the choices, but be sure to read all the choices to make sure an "All of the Above" choice would actually be correct.
If you don't know the answer right off, you can probably eliminate a couple of the choices as obviously wrong. Of course, choosing between two or three answers is better than choosing between four or five. If you've studied, you should have the confidence to narrow down your choices and then make the best educated guess you can.