[boricj]

Practice. Lots and lots of practice. There's no way around that.

Besides that, there are plenty of resources to learn particular topics/techniques out there. For drawing people with any degree of realism, you'll need at least drawing proportions at first and then anatomy later on.

While you can brute-force it from zero on your own like I did, I wouldn't recommend it. You'll learn faster if you study it like a proper discipline.