Many colleges and universities use your scores from the ACT or SAT to determine how you perform compared to other students across the nation. Refer to the admission office web pages for all colleges/universities to which you are planning to apply.

There is no set number of times you should take the tests. A good rule of thumb is to take one toward the end of your sophomore year then use those results to help you decide what kind of preparation (if any) you need do to improve the score required for the college, university or scholarships for which you plan to apply. Then take another one during your junior year using those results to help you decide if you should plan to it again during your senior year. 

First, look at ALL of the colleges/universities in which you are interested to see if they prefer one over the other. Some students report liking one test format over the other. Most colleges in Missouri will take the ACT. Colleges on the coast or ivy league colleges will ask for the SAT.

You can register for the ACT at their website: www.actstudent.org. 

You can register for the SAT at: https://collegereadiness.collegeboard.org/sat/register.