Preparation for a test should begin well in advance of the test. One of the best ways to prepare for assessments is to keep up with the course and make an effort to learn the material along the way. Be sure to complete assigned reading before the topic is covered in class so that the lesson/lecture will make more sense. Review class notes within 24 hours of taking them. As you complete homework make sure that you understand all concepts and vocabulary. If something is unclear it's best to get help right away as new concepts often build on prior learning. By keeping up and studying in advance you eliminate some of the last-minute stress associated with testing.
As the previous answer states, working out practice problems and sample tests will help you gain confidence and be prepared. Just reading over questions/answers and solved problems is not as effective as answering the questions and solving the problems yourself. If you have a friend or group to study with, quiz each other and talk through problems together.
It would be worthwhile to check and see if your school offers test-taking workshops or study skills assistance of any kind.
Test taking is a skill, just like any other skill and it's not all about knowing the subject content by heart. Of course, it is important to know the subject matter. More importantly one should be accustomed to taking tests and quizzes. Often, even though we know the subject matter, lack of enough test practice can be a demotivating factor and cause our performance to fall. One good technique to overcome the "fear" of quizzes and tests is to simulate the same environment at home and take sample tests and quizzes. Find a peaceful place at home or in school, get some practice questions or sample tests and try to do everything the way you will do during the tests (i.e., keep mobile phone away from you, do not get disturbed by family members or anybody else, etc.). The more closely you simulate the environment, the lesser fear you will feel during the actual test or quiz. You can start small, with tests of very short duration and progressively go to full length tests/quizzes. Also have confidence in your knowledge and efforts you have put into studying the subject.
Hope this helps.
Some people do poorly on tests even if they know the subject matter very well. Here are a few techniques that help some of them do better.
1) Try using paper (you might need two or even three pieces) to shield everything but the item you're working on. Why this works: you may be someone who benefits from eliminating distractions.
2) Try reading each item, moving your lips ( but not making amy sound). Why this works: reading aloud activates the language area of the brain, which can help us understand the question and frame better responses.
3) If your test is on paper, try following the words with your finger, or underlining each word with your pencil as you read. Why this works: it does the same thing as in number 2 above, but with the other side of the brain. This works especially well with math word problems.
Be sure to study right before you sleep the night before, get enough sleep, eat a meal with protein and complex carbs for breakfast the morning of your test, and drink plenty of liquid in the couple of days leading up to a test. Dehydration, hunger, and lack of sleep take a huge toll on test takers.
Whatever you do, don't fall into the trap of thinking you can't do it or that you're stupid. You're not!
Good luck and hang in there.