I agree with Spiritdreamer, find yourself an anatomy class. It helps so much to have an instructor who can show you and to whom you can ask questions. If you can build muscles on your skeleton - do it! I’m so jealous… Learn the muscles - their shape, origin, insertion, and fiber direction, and voila! you have their actions. I TA’d for one of the best Kinesiology instructors and she had one really simple, really good rule of thumb for determining muscle actions - muscles get shorter. That’s all they do. That’s all they can do. They bring the origin and insertion closer together. If you know where they attach and their fiber direction you can determine what they do. 
And… touch your skeleton a lot. Move it around. Look at everything closely. It helps so much to have one around. The skeletons in my anatomy classes were my best friend. Oh… and another thing - if you can find a willing participant - draw the muscles on them (with washable markers, of course). Don’t try to learn more than a small group of muscles at a time. Pick, say, the shoulder and learn the muscles there. Make sure you feel familiar with them before you move on. 



And, did I mention expressive?


