Maintain a high GPA. Though admissions officers won't be breaking down your door just because you have a near-perfect GPA, having excellent grades will keep you in the running for admissions to the college. Most Harvard applicants fall within the top 10-15% of their class, so you should aim to be within this range to be seriously considered. If your GPA is well below this range, the other parts of your application will have to be incredibly impressive to make up for it.
Maintaining excellent grades in high school tells admissions officers that you are a hard worker and that you will be able to keep up with the rigor of the courses at Harvard. You can show that you can stand out in other parts of the application.
You should demonstrate excellence in all of your academic subjects. Though you can be stronger in some subjects and weaker than others, you should be an exceptional student in almost every class.
Don't be discouraged if your GPA isn't quite as high as you wanted it to be. If your grades have improved from your freshman year until the time you apply, officers will be impressed, and if there was a dip in your grades because of a personal situation, you can explain it in your essay.