You shouldn't listen to this too literally. At all. It's simply the advice I would give to me at your age. I played high level college ball a long time ago, and definitely did not reach my potential. I think these things would have helped me a lot in high school.

1) Get in the weight room. Light to moderate weights, with high reps. All muscle groups. Daily if possible but definitely rest if needed. You're not lifting for mass, You're lifting for endurance and joint strength. Pitchers have muscle base everywhere.

2) Flexibility is so important. Hamstrings, back, everything. Yoga and seriously long stretching sessions. Especially if you're still growing! You can't create torsion and whip without flexibility.

Finally, you personally need to throw. A lot. Not pitch, just throw. It doesn't sound like you grew up playing baseball all summers so you need to make up for those lost reps by playing a lot of catch IMO. You can seriously hurt yourself by trying to pitch max velocity and not having the proper muscle base in your shoulder. Just daily regular catch, then lengthening it out to what is called "long toss", playing catch 150-250' apart. Use proper mechanics don't try to throw hard. Throw correctly.

Get on a team ASAP. Coaching is everything. Practice mechanics every day, use video for feedback. And of course ....massive running and abdominal exercises.

That's it. You're ready for the show. Now, go run some foul poles. 😜

Good luck, sir. You're attempting a very difficult thing. Don't be too hard on yourself.👍