What can a teen do to significantly improve their future?

Don't do drugs, don't get pregnant, finish your studies, travel, listen what older people have to say, move your body, read HN.

Without any other context, the best I can tell you is this: Do not take on debt, and never use tobacco.

start a habit of writing daily and once a week publish one of the articles that you wrote. This habit alone is the most powerful thing for your self learning.

2nd. learn Anki spaced repetition to learn more effectively.

Have fun and don't think about tommorow