Many, though not all, colleges require standardized test scores as part of the application. For schools that do ask for test scores, almost all accept either the SAT or ACT. You might want to consider taking both tests, as some students perform markedly better on one or the other. Both tests are offered several times throughout the year. For information and schedules see: SAT and ACT
Some schools also require SAT subject test scores. Usually two are needed; there are a few schools that ask for three. It’s best to take subject tests as soon as possible following completion of a relevant course. For example, if a student does well in US History or Biology in 10th grade, he/she should consider taking the corresponding subject test in May or June of the sophomore year. Even if specific college requirements are unknown, it’s a good idea to take the test when the information is fresh.
There are some colleges for which standardized tests are optional. For a list of these schools, see: fairtest.org
Advanced Placement (AP) Tests
AP tests are not required for college admission. Some colleges do use results of AP tests for course placement and/or college credit. Check specific colleges’ policies to determine the benefits of taking AP tests. Regardless of whether the AP test is taken, enrolling in AP classes in high school might be beneficial in college admissions under appropriate circumstances.