Attendance, make sure you make it to school and class on time.
Being at school is important so you don't miss content you may need to be success
Work hard in your classes, the better your grades the higher your GPA.
Do more than just school (colleges/universities/scholarship boards want to see a well rounded student).
Volunteer/Community Service
Extracurricular activities
Part time job
Participate in college fairs, leadership activities, etc.
Figure out what you want in your go on pursuits
College/University - What program/major do you want to study?
Military - Do you want to enlist or seek out officer training? What military occupational specialty (MOS) are you thinking of doing?
Workforce - is the job sustainable? Can you be promoted? Will it cover cost of living?
Sign up for the SAT School Day exam (or an alternate date for ACT or SAT - link)
Look for scholarship opportunities (many national scholarships are open all the way down to Jr High age). See Scholarship Page: search engines (bottom of page)
Have you passed all required credits? If not, talk to school counselor to make a plan for credit recovery.
Summer:
Retake/take College Entrance Exams if you want that on transcript for colleges. College Entrance Exam Link
Try to go and visit college/university campuses you are interested in. Most campuses offer tours (register/arrange in advance).
I believe it's important to get on campus and get a feel for the area and student life.
Senior year you only get 1 excused day for campus visits so this is a great time to explore.