ACT and SAT testing is normally done end of Junior year beginning of senior year in high school. You can take it as many time as you like. I suggest starting early your junior year so you have time to take it as many times as you need. The Princeton review offers a great preparatory class for the tests; there is also a fantastic book you can buy that contains practice tests.

Depending on the college you attend, they may require one or the other. Most colleges in the south prefer the ACT while colleges in the north prefer the SAT. Look up the scores they want, also see what scholarships require, and then set your goal.