Hi there. It's quite common for people to feel disappointed with their test scores, no matter what they are.
First, some questions I'd like you to answer to yourself honestly: did you study as much as you think you should have? Did you put in enough time preparing? Are you satisfied with your level of effort? Do you think you could have or should have tried harder? These are important questions, ones that you should think about if you're going to seriously try to raise your score.
Raising your test scores requires a good amount of work – these are not easy tests (namely the ACT and the SAT); they are, in fact, designed to be difficult and challenging for almost every single person who takes the test. They need to be hard enough that even the best-prepared high school senior will struggle with a few questions (otherwise, the test is flawed).
That said, you probably want to know how to improve. The plan is relatively simple; the implementation, however, is more difficult. You first need to buy the best ACT-prep materials possible, which should always include official test materials. You may use non-official study material to learn strategies and to practice, but you should always, always use only officical material for practice.
You then need to work your way through the materials methodically. Focus to the best of your ability while you are learning, and take notes. When you take a test, make a note on your test paper or in a special notebook (if you use a special notebook, keep in mind that you won't be able to do this during your official ACT) to keep track of the questions that caused you trouble. When you're finished, you absolutely must review and figure out what you did wrong.
You have a good GPA, so you may have a good chance of getting into a good school. Don't give up, and stay focused! If you're motivated enough to come to a forum to ask people how to improve, then you're motivated enough to do better on your test. One final note--be sure to take a practice SAT as well. Chances are the colleges you'll be applying to will accept those scores, too, and you very well may do better on the SAT than on the ACT.
Good luck, and study hard! Let me know if you have any other questions.