For your work specifically I would definitely follow the earlier advice of anatomy.
Also proportions seem like they might be a bit of an issue as well. When you're just starting out doing art it is a critical phase where you have to make sure you don't pick up any bad habits. Also look through several tutorials, any tutorials as long as the person producing them has experience and good artwork. The thing with art is that the more you learn the more you can apply, you may think that you don't need to know about perspective too much since you're drawing figures in plain backgrounds but if you learn perspective it will help if you want to do foreshortening. Or if you learn colour theory you'll know which colours to use that make the subject matter to pop out more.
I'm not sure where your goals "artistically" are since drawing for a porn community usually means you're happy if can just draw hot girls doing hot things or if you want to branch out more.
Anyway tl;dr figures and proportions are where you should put your focus, then when you are comfortable get into colour theory and lighting. You should also look into tutorials on HOW to use your tools, whether you want to focus on digital tools like photoshop or traditional art.
There should be a thread somewhere for artist on this forum with a bunch of handy links. here are 2 youtubers who it think you'll benefit the most from watching.
www.youtube.com/user/Sycra specifically these 3 to start you off:
www.youtube.com/watch?v=oKFfSl-EBfI www.youtube.com/watch?v=iBadnTAzQFc and
www.youtube.com/watch?v=qxZbsLBd3oU)
And this guy
www.youtube.com/user/ProkoTV its mostly figures and how2draw'em so it should prove helpful