Since I'm in graphic design, mine was a little more clear cut about what to include, though I tailored which design pieces to show off for specific job interviews.
First, a resume. An up-to-date and professional looking one. My favorite book for resume writing is: Heart & soul resumes : 7 never-before-published secrets to capturing heart & soul in your resume by Chuck Cochran, Donna Peerce. See if your library has a copy. Very good info there.
Second, a transcript is great to include. High school isn't required but if you took special professional courses or took a college prep program, than feel free. College is a must. If you're a current student they'll usually print one out for you for free.
Third, letters of recommedation. Not necessarily for a specific job, but ask a few teachers or professionals to highlight your great skills and personality. My resident advisor was a great help with one of these.
Fourth, any good school work that's relevant. This could be special projects or group work and also any papers that you've gotten good grades on. To highlight a project, write a summary page telling what the project was, how you worked on it and what you learned. Also relate how you can apply that knowledge to your future profession.
The content isn't as important as what you know and say about each item in there. Each item needs to have a reason as to why it's there. In an interview situation, you'll be talking and explaining the portfolio to the interviewer while they page through it. You can point out good things about each item, but keep it brief. The whole point is to interact with the interviewer (think Show n' Tell) and explain fully your professional strengths and acheivements more than with just a resume alone.
I personally like them because you can actually point to something and say, "See what I've acomplished," rather than just sitting there and using words only to convey your talents.
Take the time to really make something that you might use later because it will really make you stand out against another job candidate that has a similar resume but no portfolio. Oh, and put it together with a plastic portfolio book and not just a binder. These are narrow bound page protectors in a plastic cover. Avery makes some at reasonable prices. They just look a lot more professional than a plain binder and they're smaller so they fit in a briefcase. Good luck!