First, research the colleges you want to apply. Not all schools require the ACT. If they require the ACT, then in your junior year take a practice or this test to see how you are prepared. This will give you time to practice areas of concern by take a guide prep course or using prep materials through library or online.
Find ACT practice test workbooks on amazon.com or see if anyone can lend you used ones. The best way to do well on the ACT is to take practice tests, check the questions you missed and learn from those misses in order to improve on the next practice test.