What year are you in high school? What math are you currently taking?
If you are not finishing all the questions on the ACT, or struggling/rushing to finish them, the SAT is probably a better test for you. Regardless of the test you choose, daily focused practice is necessary. In addition you have to do timed practice tests. You can do practice section tests (timed) but you have to do at least one full timed test too. These tests are like anything else, one needs to practice to get results.