I'm pretty good at environment art and modeling/sculpting in general, but when it comes to anything human related I actually have no idea what I'm doing...
What's a good resource to start learning anatomy/character art in general?
anatomy books
google body part and try to copy it
Study the skeletal system and know where the outer layer muscles begin and ends. Some books I recommend are,
Albinus On Anatomy by Robert Beverly Hale, Artistic Anatomy by Dr. Paul Richer
Figure Drawing by Michael Hampton
You'll have to learn how to break the muscles/body parts into basic shapes. You should can look up George Bridgeman as well.