So when I was in High School I wasn't able to take any art classes so I did not have a portfolio when applying to colleges yet I ended up graduating with a degree in Studio Arts. I did not go to a school specifically for art but if that is your goal having an art portfolio is definitely necessary. I think the more experience and classes you take the more art you will create and then you can choose from these pieces to create a portfolio that describes who you are as an artist. If you don't have to opportunity to take art classes you could also look for an internship that would allow you to be creative and work with other professionals in the field. The important thing is that if you have a passion for this, get started! Sign up for classes and talk to any of your teachers or people in your community who might be able to get you started on the right path.