Applications – By now you should know what schools you're applying to 5 or 6 ideally. You've had a few months to look over the various applications and think long and hard about your essay. Give yourself ample time in November to complete all the applications and get feedback about your essay. Have parents, teachers and other adults in your life read your essay and try to keep an open mind to their constructive criticism.
Financial aid and scholarships – Deadlines for financial aid and scholarships are still further down the road, but the sooner you apply the better. Knowing what kind of aid packages are available will help you weigh the cost of different schools.
Testing – November is still within the normal window to take your SAT and/or ACT, but December will probably be your last opportunity to do so. If you haven't taken or would like to retake either one, make sure you register now for the December test. Retaking in the spring will be too late!
Grades – November is also a critical month for your high school classes. The semester will end in mid-December, so if you need to improve any grades, now is the time to get serious. Even if you are accepted to a school, the decision can be reversed if your grades take a nosedive.