When I attended school for design years ago and applied to other schools some of them actually put out a list of things that they wanted to see from applicants. You could check and see online - I am sure there are some references - however a lot of them wanted to see examples of many different times of media. These days there will be an emphasis on a lot more digital programs, however , it is always great to know how to use old fashioned tools - i.e. pencil and paper, pen and ink, guache, water colour, acrylic etc and even experimenting with photography. Try out different media - even take a subject and do it in all of the media you have access too and see how you work with each one. You'll find out the ones that you like the most and it may even change your focus. Try different perspectives as well. Also - if you can find a "life drawing" class somewhere near you - attend some classes, they are great at helping you build foundation for your drawings - and if you should end up in animation or illustration will give you incredible strengths in modeling.
There are many crucial steps to putting together a solid art portfolio. It might seem overwhelming at first, but incorporating the following ten principles into your process early on can mean the difference between an adequate art school application and an outstanding one:
- Start building your art portfolio early
- Get familiar with the programs and art schools you’re applying to
- Create original work for your art portfolio
- Experiment with your portfolio
- Include work in your portfolio that shows your strengths
- Consider including works-in-progress in your art portfolio
- Portfolio curation is everything
- Effectively document your art portfolio work
- Attend National Portfolio Day
- Think about the big picture beyond your portfolio
Other interesting sources: https://www.studentartguide.com/articles/how-to-make-an-art-portfolio-for-college-or-university
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.