I think, before you get into learning how to render and shade and draw clothing, you really need to sit down and study anatomy in detail. Probably the best people to look at are Andrew Loomis and Vilppu, and I think you should also try and gather up a nice amount of photo references. Here are some links that will help you:
The best thing to start with is Fun with a Pencil. You may be like "wtf this style is old as shit why does she expect me to learn from this". It will help you get used to drawing and 3d shapes, and get used to proportions in general no matter the style. Then pretty much just go in order with the books.
Glenn Vilppu is another godly teacher when it comes to learning anatomy and stuff. Heres his drawing manual, but I STRONGLY suggest looking for a torrent of his videos. He goes way into detail on each body part in his anatomy videos, and then his gesture videos are super helpful as well.
Another great teacher
And here are references. Before you click, don't be surprised because the ads are kind of..suggested. So um, dont open this in a public area
Male naked photo references
Female naked photo references
Humans in Motion photo references
Tutorials. the second one is more for digital painting, but the first one is fantastic as well