Dear Kelsi --
I am a high school teacher and have advised a number of students at our school who are interested in the service academies, including several who have gone to the respective Prep school (mostly USNAPS) before going on to the Academy. A few thoughts, building off what others have said. Most importantly, as others have noted, your ACT right now would almost certainly not be competitive for direct entry. So, what can you do? Here are some suggestions:
1. If it's financially possible, think about trying to take the SAT at a date early in the fall. Some students do significantly better on the SAT rather than the ACT, and it is worth seeing if you might be one of those students. Obviously, you would not want to go into the SAT "cold." Go to the College Board home page and you will see there are a great deal of resources available to help students prep for the SAT. If you do think you will take the SAT, get familiar with the format as early as you can and start practicing, practicing, practicing. If you feel you are lacking some of the underlying math skills that the SAT focuses on, maybe you could ask a math teacher at your school to recommend a textbook that you could use to help you brush up on that material. If you have access to a reasonably high speed internet connection, over the summer why not access the famous "Khan Academy" series and focus on your math skills.
2. If you feel like it is not feasible to try to take the SAT, then just focus 100% over the summer on working on your ACT. Look on the ACT home page for free resources available to help students, and see the above advice about borrowing a math textbook over the summer and working with the Khan Academy online courses. Also, remember that you do NOT need to pay for an expensive SAT or ACT prep course to prepare. In addition to many free resources put out by the testing companies, you can buy a test prep book for $20 or so (and probably get a used one even cheaper, or ask around at your high school to see if there are graduating seniors who might have old Stanley Kaplan or Princeton Review test materials). Practice can really help.
3. Consider contacting the West Point admissions office and asking to be put in touch with a minority outreach admissions rep. You have a great attitude about not wanting any favors, but it is a fact that the military academy considers Native Americans an under-represented minority (you'll sometimes see the acronym "URM"), and is interested in helping get them to USMA and set them up for success at USMA. This is a personal decision for you, of course, but remember that West Point also gives preference to applicants from underrepresented geographic areas (it is easier to get in as a candidate from some states than from others) and to recruited athletes.
4. Start thinking about USMAPS as a possible option. You cannot choose to apply to the Prep School -- the Academy makes that determination -- but currently with your math score you would be considered not academically qualified and the only realistic option would be to attend the Prep School for a year first. It is a great goal to work to get your math score into the academically qualified range, but psychologically you may also want to recognize that USMAPS is another way to attain your ultimate goal of attending West Point.
Good luck to you!