Because every art school has a different set of requirements for portfolios, you should first make a list of the schools you want to apply to so that you can research what you must submit and how.
You will be expected to submit between 10 and 20 pieces in your portfolio. Make sure you select the pieces you feel showcase your talent and technical ability. Remember that the pieces you select should be original works, not cartoons or comic book characters, singers, and so on.
Every art school will require that you include a number of "observational artworks," which are drawings or paintings in a traditional style of things from real life, like figure drawings, landscapes and still lifes.
You will also be asked to include some examples of "personal art," which should be artwork you have produced on your own initiative, outside of the classroom. It's meant to show your interest in use of materials, subject matter and concept. Personal art can be done in any medium, such as drawing, painting, photography, mixed media, digital art, film/video, ceramics, sculpture, animation or performance.
Depending on the school you are applying to, you will be asked to submit your portfolio in slides or as images on a disc. Sometimes schools will allow you to send your original works (if they are drawings and sketches) as long as they can be shipped easily. But you should always include a back-up of the work, in either slide or disc format, in case the original becomes lost. Make sure you put your best work first.
Spend some time taking the best quality photographs that you can. The work should be well lit and fill the camera frame. Only submit slides that are clear and in focus.
Label the slides with your name, date, title and dimensions of the work.
Include a separate typed slide description sheet.
Place the slides in either a clear contact sheet or a carousel (depending on what the school asks for).
Nowadays most schools will ask for your portfolio in digital format on a disc.
The images should be in JPEG format and you should double-check that they can opened on both Windows and Mac platforms.
Include a color printout of the work as well as a copy on disk format with attached list of instructions and programs used.
If your work is on video, you should adhere to each school's time limitations and compatibility requirements.