Use the Deku flower nearby and you should be getting it in no time!
He's quite selfish and all he wants is Majora's Mask back.
When you start the game, there is some interesting information the Mask Salesman will share with you, but not much to do here for now. Confused? Read each mask's section below as there are many details to help you on your way. Also, don't spread yourself too thin, if you find that you were not able to get a couple of those masks in a 3 day cycle, play the Song of Time and start again. Plan these tasks accordingly, if you save the Bomb Lady (midnight on 1st day) and get the Blast Mask, you wont be able to get the Keaton Mask the third day on the same cycle. However it will take you a couple of 3 day cycles to get them.
You will need to do a series of intermediate to very difficult tasks, depending whether you're familiar with Legend of Zelda: Ocarina of Time. The following is a list of masks you can get before you even go to the Woodfall Temple.