Good afternoon Damien,
If you would like to make brushes start with a white background and make some marks, or scratches or even type in some text. You should adjust the contrast higher so it is blacker but this is not critical.
When you are done, select the image or the part you wish to make into a brush, from the brushes' palette choose 'define brush'

You are all set. You now have your first custom brush.

Yo