You should start developing your portfolio by first doing a self- assessment. Evaluate what you have to offer and what the best ways are to market your assets. You should decide which skills and experience you have will relate to the needs of the interviewer, or what you would most like the interviewer to see. If you don't have much work experience- most students don't – you'll need to mine your school work, volunteer work, and hobbies for evidence of the relevant skills you know you have. For example, if you did event promotions for a campus organization, include a copy of flyer or poster you designed.
A portfolio is a visual archive of your artistic accomplishments that demonstrates your ability and potential as an artist. Students are encouraged to present a series of images that represents current interests. Also, the portfolio should document additional work that demonstrate an awareness of formal visual organization principles and experience with a variety of tools, media, styles, and approaches. Every visual arts college has its own specific portfolio requirements. You should research the portfolio requirements for each school to which you are planning to apply.