Check the harvard admissions website and collegeboard.com, it's all on there. The only requirements (aside from high SAT scores in both the SAT 1 and three SAT2s as someone has already mentioned) is that you should be taking a 'very vigorous course', ie. doing as much as you can and getting the highest grades you can without killing yourself.
As for the engineering side, Cornell is brilliant for Engineering. My cousin is majoring in engineering there. Cornell also has a 30% admit rate or something - better than 10% admit for Harvard.
And, if you aren't getting all A's at AS, I don't think it would be advisable to apply to Harvard, because there would be a high likelihood of rejection.
Then again, you never know what might happen...