What should I start doing from now?

It depends on what you are planning to major in. Harvard will probably tell you the prerequisites you need for doing your major.
To actually get into Harvard, you should probably study hard. Maintain a high GPA and don't fall down on your studies. If something is giving you a hard time, don't just shove it to the corner because you don't like it. Work hard at it to improve yourself.
You should also NEVER procrastinate. Good luck in getting into Harvard in about 3-4 yrs!