It's tough, and you'll probably need to pull your GPA up. But you might be able to pull it off. In case you don't, though, you should apply to safety schools, as well as a few other reach schools similar to Harvard (Yale, Princeton, etc.) because you might get into one of those if you don't get into Harvard.
Also, Harvard isn't necessarily good at everything. So I am a little concerned about your statement that you are willing to change your career rather than changing your school to one that will be good for your intended career of architecture or graphic design.
Next, if you can get into Harvard, financial aid will not be an issue. You will have to fill out a bunch of forms, but you will get a lot of aid if your parents are middle class. If they make less than $180,000 per year total, they will only need to pay 10% of their annual household income. And if they make less than $60,000 (I think that's the threshold), they wont have to pay a cent, as Harvard will pick up the tab. (This is a relatively new policy.) You might have to work summer jobs to pitch in a few thousand, but your parents won't have to pay anything. Harvard doesn't do specific "merit" scholarships; instead, everyone who gets into Harvard is automatically eligible for financial aid grants on the terms I indicated above.
Finally, try not to fret too much. If you don't get into Harvard College, it's not the end of the world. Seriously. As long as you worked hard in high school, you will get into a very good college. And then you can always go to Harvard for graduate school; many of Harvard's graduate and professional schools are actually easier to get into than Harvard College is.
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All that said, if you want to get into Harvard College, here are the main things you must do!
1. Get good grades. Straight A's are really the best. But a mix of mostly A's and a very few B's is OK, especially if the B's are in tough AP/honors courses. So, try to get straight A's, but if you get a B, don't kill yourself over it--just work harder. Try to stay in the top 10% of your high school class (top 5% if you can).
2. Take challenging classes. Honors, AP, etc. Don't take soo many at once that you overload yourself and get a C. But take as many as you can take with good grades.
3. Clubs/extracurriculars are important, but there is no "magic number" of activities that you need to do.The most important things are commitment (do the same activities for long periods of time) and leadership (being president, or treasurer, or committee chair...). And you should have at least 1 or 2 community service activities, but your other activities can be clubs, sports (especially varsity) or student council, etc.
5. Get as high scores on the SATs (or ACTs) as you possibly can. Study and retake if necessary. Again, you want to be in the top 10% if you can. If you take the SAT, you'll need a few SAT II subject tests in addition to the SAT I.
6. Think about what teachers you might ask for recommendation letters, because you need good ones. Try to take more than one class with them, so that they get to know you well. Or the reverse: in your senior year, ask the teachers you've taken the most classes with (or done the best with) to recommend you. But if you start thinking about that now, you can perhaps plan accordingly.
7. And of course write a good essay that you have proofread and reviewed by others.