Putting Action into Still Images
Once you get used to basic-stance figures, it's time to put some movement into them.
Start by keeping the body straight but putting the arms in different positions. Now do it with the legs, too. A lot of artists use a thing called an action line to base their character's movements on. this is just a single line, curved, squiggly, or whatever upon which the rest of the figure's movements will go along with. I don't use it, too much of a hassle, but it does help some people. Once you have practiced with arms and legs, move onto turning the head. You will have to know and practice with the different angles of the face.
Now it's time to learn about fore-shortening. Fore-shortening is a trick artists use to make something appear to be in the foreground or background or appear to pop out at the viewer. It calls for deforming one or more parts of the figure to achieve that effect. For a figure reaching out at the viewer, make a wide-open palm much larger than the figure's other hand(unless he's reaching with both). Make sure some of that hand covers up part of the body to kind of sperate the movements of the two.
An important thing to know beforehand is exactly how the certain parts of the body move and what are thir limits. Never make the action look like it took the figure to endure more pain than William Wallace's torture in order to get in that pose. On the animation side, never give a movement a too mechanical look. Always keep a curve or jolt to the movement, rather than a straight beeline from point A to point B. If the character's reaching for a can of soda, have hime lean his body forward and his other arm a little backwards.