If you are one of those students who knows exactly what you want to do, always be ready with a back up plan, just in case. Not to be negative, but you never know what can happen in the future that might prevent you from entering your planned career field. Your career interests could change, unexpected family or financial circumstances may arise, the classes for your major may become too difficult, or if you are required to attend graduate school or a specialized program after you obtain your four-year degree, you may not do well enough on upper-level standardized tests, like the MCAT or LSAT, to gain admission to the program. Don't worry if your career plans change, as there are many other options out there for you.
If you are one of those students who has no idea where you want to work part time, let alone what field you want to work in after college, evaluate your strengths, weaknesses, and abilities to help you determine what career path you should follow. You can visit your school's career center, take a career assessment test to help determine which fields interest you, or visit a career fair. Once you have picked a few careers that interest you, try to find someone who actually works in the field to talk to. You may be able to have this person as a mentor, and/or you may be able to shadow him/her during work. Other opportunities to find out more about your chosen career fields include internships and co-op positions.
Regardless of your future career plans (or lack thereof), you should still apply yourself in college and take advantage of any available opportunities to explore your career options.