I'm assuming you mean a year from when you would enter Harvard (or any other college or university) which in most cases would be late August.
First of all, I don't know anything about your background. I don't know whether you've every taken an SAT or ACT before, or whether your first language is English, or what kinds of high-school courses you've had access to. I don't know what your grades have been up to now (I'm assuming you're in junior year). Without knowing those things, it's difficult to know exactly how to answer your question.
Secondly, the idea that if you simply "prepare," you'll get in (to Harvard or to any university with less than about a 70% acceptance rate) is not a given. For Harvard specifically, whose acceptance rate overall is between 5 and 6 percent, NO ONE can assume that she will be accepted. The best she can do is be a competitive applicant among the other 35,000 or more students who apply to Harvard College for their bachelor's degree.
That said, to be competitive, you'd need to be taking your school's most demanding courses in every "solid" academic subject (a first language, which is usually English; a second language; mathematics; science; and social sciences/history). You'd need to have taken a class in each of these subjects from 9th through 12th grades.
Whatever classes you're in now, do well during the rest of the year. Do REALLY well. Make mostly A's. Make all As if you can. Next fall, take the more or most demanding level of that class and be sure you can still make an A or B.
For Harvard specifically, you'd need to take either an SAT or an ACT and two SAT Subject tests. (You could take a third subject test if there's a third that you know really well). If your first language is not English, you should take a TOEFL (Test of English as a Foreign Language) exam, or another exam called the IELTS if the college accepts it. This is necessary to further prove your English proficiency, since you will be studying completely in English.
In addition, get involved in your school or community, in any activity that is close to your heart or that makes you happy. In this activity, it's great if you can go beyond simply participating but also help the group in some way that "makes an impact:" where you motivate others, where you fundraise needed money, where you play a role in a theatre production, or where you tutor younger kids and help them make better grades: these are only a few examples.
As the other commenter mentioned, document all these extra things that you do: make a resume.
Talk to your high-school counselor IMMEDIATELY to help you plan how to do all these things at your specific school.
AND, it's important to talk to your counselor because she or he can help you develop a list of colleges where your academic and personal background fits the typical student there, and can talk to you about possible financial-aid needs. In short, your counselor knows where students from your school have been accepted in the past, and can give you an idea of how your qualifications fit into that pattern.
So the take-away here is:
- Find out what the admission requirements are, at Harvard or any other college;
- Talk to your counselor about how to attempt to meet these requirements for Harvard or any other colleges
- Follow these steps (make great grades, prepare for necessary exams and take them this spring or next fall before January)
- Think about what's important to you in a college: small discussion classes? being in a competitive or a more collaborative student atmosphere? being among students similar to, or different from, yourself? does your college offer what you want to study?
I hope this is helpful. Think wider than just Harvard--this is true for every student, not just you--and come up with a list of 10 - 15 carefully selected schools which include some with higher acceptance rates (over 50 percent). Talk to your counselor and get personal advice based on your credentials.