I bought my first anatomy book in junior high and started practicing with that, then I started buying other anatomy books that covered specific styles like dynamic figure drawing or comic book styles. Basically you start by learning what's on the inside (bones and muscle groups) then you practice drawing what's on the outside (developing your own style).
Photo references, stock images, and people-watching is very useful too.
When I started out I used basicly any drawing or photograph that caught my fancy. I copied a whole lot of drawings from my favorite illustrators. I looked at myself in the mirror to study facial anatomy a lot. I remember going around drawing a whole lot of my classmates ears to figure out where all the creases went. But at that pont I only wanted it to look good and never actually thought that I had to learn about anatomy. When I got older I went to some art schools and did a whole lot of life drawing. And now I'm also doing studies from anatomy books.
I Google reference pictures, draw people and clothes from department store catalogues I get in the mail, and do life drawing. I also reference myself. Last year, I got a book of anatomy which was really useful.
Anatomy - life drawing classes, anatomy books, hours upon hours of study (which I am nowhere near done with) Style - People and things that inspire you, learn about art history and pull from techniques and styles that inspire you, also look at your favorite artists on DA and elsewhere, and look at the art produced by concept art companies you might want to work for one day
I think in the end it's all about studying and expanding your knowledge of not just anatomy and skill, but of the history of art and the many things going on right now that affect the industry.