I am still a senior in high school, but I have just applied for college as a history major. If I want to go to med school how will I be able to finish my prerequisites in time?
It'll be no problem at all. Surprisingly, majors can be as little as 30-45 units (Science and Engineering are typically more). GE is around 30 more units. That leaves 45-60 units for you to do whatever you want: a second major, a minor, prerequisites for grad school, etc.
As long as you take: General Bio 1 and 2, Chemistry 1 and 2, Organic 1 and 2, Physics 1 and 2, English 1 and 2, and possibly 2 math classes and 2 upper division bio, you will be golden. If your school is like most, that's ~40 to ~60 units (some classes are 4 units, some 5, and others 3). On top of that, some of these courses (e.g. English) will count toward your GE.
Sit down with an adviser and plan it out.
You are not taking into account the changes that are coming to the MCAT in 2015 or the possible changes in medical school admissions that may accompany the new MCAT.
To the OP: Go to this website (https://www.aamc.org/students/applying/mcat/mcat2015/) and familiarize yourself with the changes since you will likely take the new test. Plan your classes accordingly and be flexible in your major selection. And definitely talk to your new university's premed advisor (assuming they're competent). Also, be cautious with the advice given on this website.