Review the high school graduation and "A-G" requirements (found on the Graduation Requirements and College Information pages).
Colleges/Universities only accept "C-" or better grades, which means that if you earn a "D" grade you will need to re-take the class for a better grade.
Review the Course Catalog and talk to your counselor to help you choose classes for 12th grade.
Start thinking about who to ask for a letter of recommendation. If you are applying for programs, private universities, and/or scholarships you will need letters of recommendation. Some schools are very specific as to who should write the letter, and it may depend on your major. Make a copy of this letter of recommendation request form to give to your recommenders.
Take AP exams if you took AP Courses. If you get free/reduced lunch you qualify for a fee reduction.