Don't bother until at least your probably third year of college. You won't be getting anywhere near the core classes for your major until that time, so there's no point spending money on a laptop that'll be ready for replacement by the time you need it.
The first two, maybe even more, years of college will just be a repeat of high school with freshman english classes and gen ed type stuff. You wouldn't need much more than a netbook that can do some basic word processing for those. Once you've actually declared your major and been accepted into the degree program you can go out and get a new laptop. By then you'll probably have figured out that a laptop for college is a much better idea in theory than practice. Few lecture halls will have much more than a handful of electrical outlets, which will have fierce competition, there probably won't really be room to set up a laptop, taking notes will prove difficult even for a very fast touch typist (there's a reason court stenographers use a kind of shorthand) and I can pretty much guarantee you'll end up screwing around on Facebook or some similar site during lectures instead of listening to the lecture. You also have to figure that you're adding an extra 2-3lbs onto the weight of any books you buy (and another tip: Don't buy ANY books for your classes until at least the second week of class. Half the time you won't even use the book, so even having to spend more on new copies because the used are all gone, you'll come out ahead easily) for your classes that day. You should expect to be walking an average of 1-3 miles every day going between different campus buildings where classes might be scheduled, so consider lugging around 10-15lbs worth of books and then adding to that another couple pounds for your laptop.
My advice would be to get yourself a modest desktop that you leave in your dorm/apartment/frat/whatever. Do your homework on that as needed, then when you actually get into your degree program you can decide whether or not to just get a better desktop or a laptop.