Write Form Input Value

I want to have text already in the new post input text box on the write page.

I am referring to the ‘Write’ page in the administration section of wordpress. The ‘Write Post’ section has 3 main input box’s. The first being ‘title’, then ‘exerpt’, then ‘post’. It is the ‘Post’ input field that I want to have a default text value in. What file do I edit to achieve this?