You first have to go to Castle town and talk to the guy in the mask shop to get your mask to trade.

Fox Mask

Royal Guard at the edge of Kakariko Village on the way up to Death Mountain.

Skull Mask

In the Lost Forest. Enter the forest and move to the left. Hop on the tree stump
while wearing the mask and play Saria's Song. The Skull Kid will then ask Link for
the mask. 

 Spooky Mask 
Sell it to the young boy who plays in Kakariko Graveyard.


Bunny Mask

Sell it to the man running around in the Hyrule Field by Lon Lon Ranch.
Also this guy will give you unlimited money so go on a shoping spree before you give
it to him

Mask of Truth

With this mask Link can talk to the "Gossip Stones" that have the eye on them.
They will reveal helpful hints throughout the game.