Begin searching for a job before entering your senior year of college. You don't need to start putting job applications in just yet, though. Job searching for now will suffice. You'll want to get an idea of what is out there and what to expect when you are ready to start applying.
This will also give you time to study and research the companies that are out there in the field you are preparing for while in college. This will give you an idea of what requirements are needed from you in order to obtain the position you desire. You can do your research on the Internet, in newspaper want ads, and even by word of mouth. You can also schedule a tour of the company if you really want to get into your research.
Sometimes college libraries have research materials and information on companies that hire. This is more geared toward colleges that specialize in certain fields. Also look into the career center at your college, if they have one.