Some colleges require or recommend that applicants take two or three SAT Subject Tests. Unlike the SAT or the ACT, these are hour-long content based tests. Each test is on one subject and a student can take up to three in one sitting. There are 20 SAT Subject Tests in five general subject areas: English, history, languages, math and science. You should choose the tests that will best showcase the subject areas where you excel. A few other things to consider:
Check the college's website to see if it requires any specific Subject Tests or if you can choose. For example, a college may require that prospective engineering majors take the Math 2 Subject Test and a at least one science Subject Test.
Take the subject test right after you've completed the recommended classes in school because the material will still be fresh. Look here:https://sat.collegeboard.org/practice/sat-subject-test-preparation
Prepare. Try the free practice questions, download the Getting Ready for the SAT Subject Tests practice booklet or use other preparation resources.