first of all, your guidance counselor's main purpose is to help you with this. so ask them asap via email for help.
You will want to take: the SAT I, the ACT, and the SAT II (subject tests).
practice this summer (in the next few weeks) without tutoring on each the SAT I and ACT with writing. choose the one that you do better on and enjoy more.
once you've decided, figure out your current score based on your practice tests. determine your goals. if you think you can do it, set it really high to match your high GPA (maybe go for 2250-2400). if you are far from that in early practice, just focus on what you can do. if you need help reaching your goal, sign up for tutoring for the October or November test dates.
meanwhile, pick 3 SAT II test subjects you really enjoy/excel in. take their practice tests. determine what you need to improve in, study, and then take those tests either for October, November, or December (accounting for your SAT I/ACT test date).
use your test scores to help you determine a college fit for you. check websites to see incoming freshman statistics for gpa and standardzed test scores. research more into those that match your stats to see if you like them and they have the resources for you to study there in a preferred field.
the common app is up. go to http://commonapp.org/ and start your application. check websites for colleges you like for info on supplementary application materials.
see my comment here for more info on dates
if you cram right now to get test scores in, keep your gpa up, and fill out applications, along with finding colleges that suit your abilities, you will do fine.