how do you make a collage

are there any tutorials anywhere on how to make collages?

really depends on what you are going for but...

make a new document he size you want your collage.
start draggin the photos and stuff into that document (this makes a new layer for each item you drag into it).
now you can mask each layer like you wish.
try taking the lasso tool and setting its feather to like 15 draw around what you want on each layer and click the Quickmask button....
you'll see what needs to be done