You learn to animate by doing it. In the beginning I hoped it was a trick, at the end I realized that you just need to draw every frame again and again in most cases. Anyway, welcome on pixeljoint :)

I'm an animator in primary, so I know a ton.

The big key to animation is to practice a wide range of motions and to do thorough studies in how things move.

If you work in digital or 3d animation then you can just tween everything, thereby making it so you only have to animate the keyframes. I'm a big proponent of traditional "frame by frame" methods over tweened bullsh*t.

In sprites you HAVE to animate frame by frame. I've heard of some programs that can make in-between frames for you, but I doubt that it's really possible without complex AI that can recognize shapes and images with a high level of accuracy.

If you're animating something you've never done before, get references. Study how the motion works. In every aspect of art there is a fundamental method to how something is rendered.

Not to mention that a good understanding of,

http://en.wikipedia.org/wiki/12_basic_principles_of_animation

http://en.wikipedia.org/wiki/Animation

Hello and welcome to Pixel Joint. But we do have a topic for this.