Dear Carlethia,
Concentrating is only part of the answer. Being prepared is also important. The more you know about the subject, the more confident you will be on the day of your test. Work out a schedule that gives you plenty of time to study. When you come across something in your textbook you don't understand, ask for help from a parent or teacher. The evening before a test, put sharpened pencils, working pens, and anything else you need in your backpack. That way, you're not scrambling around in the morning looking for supplies. Before you begin a test, take a deep breath, close your eyes for a moment, and say to yourself, "I know the material. I am going to do well on this test." If you need to pause during a test because you're getting nervous, do that. Don't stay in panic mode—it won't help you concentrate.