Really all you have to do create a brush is: 1. Make a new canvas (equal dimensions is a good idea). 2. Draw some black and white squiggles. 3. Go to 'Edit' > 'Define Brush preset...'. 4. Give it a name and hit OK. Your new brush should appear as the last one in the brush menu, until you add some more that is.
~ As you get more familiar with that you should look into changing your custom brushes' dynamics, they can greatly change how your brush comes out.
~ Remember to save your brushes once you've added them to your list... anything unsaved can be considered non existent.