Answer:

You are a pencil.

Explanation:

Pencil is the most obvious answer for the above riddle since it matches with every sentence in the riddle. Such as pencil is made up of wood and wood lies in the heart of a tree. Pencil is used to write so many things, used to write so many words. So it will help us to write different words.

Also the more we use a pencil the more it gets shorter. So it matches with the last sentence also i.e. the more you push, the more I move and the less I am.