Students must take a standardized test as part of their application to health professions school. Each test differs somewhat in subject matter, but all of them will test reading comprehension, critical thinking skills, problem-solving ability, and subject area knowledge.
Students should take the exams after they have taken classes in the subject areas covered. For example, medical applicants will have taken General Chemistry, and some or all of Biology, Organic Chemistry and Physics before they take the Medical College Admission Test (MCAT). Be aware that in addition to testing in the sciences, the MCAT tests Verbal Reasoning and Writing skills -- if you feel like you need work in these areas, you are encouraged to take classes that will help you to build these skills.
Consult with your advisor to determine when to take the exam, and how to prepare for it. Many students take exam prep courses, but the key to doing well is disciplined study and lots of practice tests, which will help you to become familiar with the materials being tested. This can be done by paying to take a class, or through personal study.