While Harvard does not post a specific grade point average in their admissions criteria, they do specify that the majority of students admitted into Harvard graduated in the top 10-15% of their high school class. One of the best ways to increase chances of getting into Harvard is to keep your GPA as high as possible, and do extra credit or classes to increase it when possible. Some students manage to graduate with an above perfect GPA from doing extra work.
Advanced Placement (AP) classes are at a higher level than high school classes and are generally counted as college credit. Harvard recognizes the hard work and ability that goes into completing AP classes, which can help in terms of admissions. Taking AP classes in core subjects like English, History, Math and Science shows that you are a serious student who is dedicated to learning.
Most colleges and universities, Harvard included, favor well-rounded students who participate in extra-curricular activities. Join a sports team, run for student government, participate in art shows, work on the school newspaper or tutor students in various subjects to have your application represent your diverse qualities.
Harvard requires either taking the SAT or the ACT standardized tests for admissions and prefers students with the highest scores. Better scores on standardized tests can be achieved through studying, preparation and practice tests. There are also many schools and tutors that specialize in standardized test preparation.
Harvard requires at least one letter of recommendation, but if you are able to get more than one from teachers who appreciate your work, that can help in terms of admissions. Ask teachers that know you well to write your letter of recommendation, to make sure the letter puts your best qualities forward. If possible, obtain a letter of recommendation from a Harvard graduate to seriously increase your chances of getting in.