Hi @Agruent! Nothing beats drawing from observation...so if you can find life drawing sessions or classes near you, that's a great place to start.
If attending a class isn't feasible, I've found that posemaniacs.com is a great resource to practice gesture drawings (with the added bonus that the models only show the musculature under the skin--no nudity!).
Former Disney animator Aaron Blaise offers a video course on human anatomy that is really good. The anatomy books by George B. Bridgman are also super helpful (and only about $8 each on Amazon). Even looking through clothing catalogs and sketching the models is a great way to learn.
Hope that helps. Good luck, practice often, and have fun!