You should show it as early as you can. :) This showcases your ability as a professional moving forward as you enter college and then your career. Ideally, you should show 10 of your best work (to give a glimpse of your work), and rotate them over time as you improve on your skills. In high school, I showcased letterhead designs, business card designs, flyer designs and CD designs. As I entered college, I did more complex work, websites and projects. Once you enter your career, I showed more thoughtful projects, how I get to a solution, what research I did, sketches, and prototypes and finally the final product. You will learn as you go. Good luck!
Dream big, Vivian