Udemy is fine + I've had far better sales doing my own leadgen, marketing, sales than relying on Udemy.

There are so many ways to find good niches + do effective leadgen for various niches.

I'd say book hour calls with many experts + just take a deep poll of how people accomplish their leadgen, marketing, sales.

Also... very important... stick with markets where you already have affinity.

Dan Kennedy's + Bill Glazer's books will likely help with the basics.