Great question. Most schools (city, state, and universities) don't require a portfolio in order to enter the school. They will actually help you build yours as you attend. For private schools, that's another animal. They might want to see your work, to see how your compare to other potential schools. I agree with Angela, show your best work, whatever that might be. I have friends that love photography, so they show their photography work. Drawing, they show their sketches and illustrations. The best thing you can do, if you don't have anything yet, is to pick up a drawing book and start sketching. Or go to tutorial sites like psdtuts.com.
Good luck, Vivian