If I were to be able to go back to summer of 8th grade, I would research all the opportunities available at my high school, including clubs, student government, etc. The best ECs involve leadership, and you have to start early to get positions. Also glance at the classes and any APs offered, as well as any prerequisites to the classes you want. It’s really helpful to have some plan of what you’re going to do or at least try to do during your 4 years there.
Also be nice to EVERYONE. You never know when someone may be in a position of power to recruit you or appoint you, especially if your friends are also high-achieving like you.