masks are easy.
make a square or circle and fill it with black.
go to layer/layermask/reveal all.

look at your layer panel. see how a new square was added beside your original and how that square is all white?
make your foreground color black, grab a paintbrush at 100% opacity with a large tip (80 pixels), now paint over the black square or circle you made at the beginning. 

what should be happening is wherever your brush goes over the black it hides it from sight. if you accidentally hide too much make your foreground color white and paint over it again; thus revealing your black.
very handy as it isn't destructive to your pic.

all of the above would be reversed had we choosen:
layer/layermask/hide all

many other kinds of masks to choose from, this is just a basic example.
hope this helps.

Comitted to excellence through art.