Eraser, pencil and some books on animation skills. It's essential to make a good sketch before dealing with the animation by itself..

A scanner to scan your creations. It will give you the possibility to colorize or retouching your images in a graphic program.

In case you would like to make a lot of animations, a graphic tablet (some of them are affordable) becomes essential. Don't forget that you will need some training before making your first "chef d'oeuvre".

But don't get desperate if you only have your eraser and your pencil, Walt Disney has started like this....