There is one thing I forgot in my advice: you need to have a good foundation first. For example, when I cold emailed, I had many programming projects and I wrote about them in the email. But after you have stuff done, I think aggressively (but politely) "marketing" your work is important to get those opportunities.
That’s where I (fellow High School student) struggle. I’m not really at a point where I can contribute much either way.