the scores that are easiest to improve are the mid 20 range. request a copy of your latest ACT test. Once you have your latest score and a copy of the test in hand, contact a private "one on one" tutor. the tutor my son used cost $430. he saw my son 3 hours one evening and 3 hours the next day. make the appointment 2 or 3 weeks before the next test date.the tutor will pin point the areas to concentrate your studies.
once you take the test again, repeat the above process as many times as time and money will allow.