[HeadUp]

dude it would be very difficult and time consuming for someone to teach you how to use photoshop, you should have a paintbrush tool in your toolbar, you can edit brushes in a drop-down tab towards the top of the window.

i learned how to do photoshop by experimenting and by reading tutorials
try this one:
http://www.adobe.com/products/tips/photoshop.html

good luck, and trust me it'll def pay off