Study life study life study life!!! Drawing from life is the single best way to get a better grasp of anatomy. Another way to improve is to go to a bookstore or a library or something and check out some anatomy books specifically geared towards artists, and draw FROM them! Just spend a while copying photos of people, and you'll notice yourself start to improve.
What sarchale said is correct. :) Practice is the key. The site has plenty of tools to help you with that, from quick gestures to full on studies.
At the top of each page, you will find two dropdown menus called "Lessons & Tips" and "Practice Tools". If you go through each section, you will find an abundance of information, articles, and of course our drawing tools. They are a good foundation to get started with, and can complement any course or books you utilize for your progress.