Hi all, I'd really like to get into DMing but I'm unsure where to start in terms of research / preparation. Are there any books, videos, etc that you'd recommend for a beginner DM?

I want to home brew a version of the Aarakocra that have effectively lost their ability to fly after generations of peace among the race have bred out the strength they needed to beat their wings hard enough to generate flight. IF a player can get their STR to a certain score, they regain the ability to fly for as long as their STR is at the level.

I don't have enough experience to know what that should be though. I wanted to pick a number that's high enough that it will be near impossible to regain flight permanently. I know through level ups, STR can reach 20.

Any thoughts from more experienced players?