I do believe that there is time yet if you take the very next test. Sometimes it takes about a month for results to come back, perhaps sooner. If you google ACT or SAT, you will find a link to the test publishing company. There they will have a list of dates and tests for you to look at. Call your NCO or ESO and see if they are giving a test soon. They 'should' be helpful in assisting you in meeting your goals. If not, you can eat the bill and sign up for a test on your own through the ACT/SAT website, like I had to.
It doesn't matter which test, but if you are concerned about your score, sign up for both give the better score.
To prepare, both websites offer a tutorial and practice exams for a price. I signed up for the the ACT website help for 20 bucks, and got some tutorials and two practice exams. It was helpful in seeing what I needed to work on. Check your local library/NCO for a study book, and just get cracking. They should have everything you need to practice for either test for free.
If you do get selected, your college credits probably will (but might not) transfer to whatever school you get accepted into. From there you will build a degree plan in whatever you're interested in.
Good luck,
Sam