any tips on creating a portfolio through highschool?

i'm heading to highschool this september, and i'm interested in maintaining a portfolio throughout it, my school has a lot of art programs that require a portfolio to join. i feel that it is worth my time and effort on curating one but i'm not sure where to start. what kind of materiel should it be made of? what size? possibly even where to buy one (within canada of course.) please let me know either through commenting or messaging me if you have any tips about how to go about this :)

thank you for your time,

- clover.