Go to the university websites to see exactly what you will need for your application. For most colleges you can apply via commonapp, but MIT has a separate application. Don't do only science and math subjects in year 11 and 12, because you typically need a letter of recommendation from both a science/math teacher and a humanities teacher. Make your teachers aware of your intentions and try to get some good letters of recommendation.
In year 12 don't shut yourself out and only study; make sure you spend plenty of time on extracurriculars, and see about doing volunteer work. They won't care about your atar too much; a straight A report is good enough, and they will look more at your SAT/ACT scores, which you will want close to perfect, and for the SAT subject tests, you will want no less than 800. You can take the SATs as many times as you want, so there is no reason not to do one this year.
Get good at essay writing. This is so important, as you will have to write an essay in the SAT/ACT as well as multiple personal essays in your application. The SAT/ACT essay will be under exam conditions, but the personal essays are not, so you should start them very early, and get people to proof read them.
Also, Caltech is more of a research school so there are not many undergraduate students there, so it will be even tougher to get in. Don't look at only Caltech and MIT, apply to a good five or so schools at least, such as the ivy league schools or Stanford, because often the case is you will miss out on one college and get into another. Just remember that a lot of students with 4.0gpa and 1600 SAT scores miss out because they were lacking on other parts of the application, so make sure you are well rounded. That's not to say don't get perfect SAT scores etc, but just don't think that that is all they are looking for.