Venetian masks are typically made with papier maché or ceramic, which makes them fragile and/or quite heavy.
If you want to start with something easy, I recommend getting a plastic face mold from Hobby Lobby, or a similar craft store.
Some of the best paper-mache masks I've seen are from http://www.goblinart.com/ - she uses a mold to press the paper into a shape, then paints it after it dries.