There are a number of easy ways to create the hook itself, depending on what sort of materials you prefer. If you're making a hook for a child's costume and want to create something sturdy but soft, make a cloth hook. Draft the pattern on paper by sketching the hook shape, then create an outline. Cut two of these from a silvery fabric (or spray-paint some felt), then machine-sew using a running stitch. Turn the hook inside-out and stuff tightly with cotton batting.
Another good option for an easy soft hook is craft foam. Craft foam is stiff but soft and durable, and is easy to paint in metallic colors using craft acrylic paint or spray paint. If the sheets you have are thin, cut the hook shape from two layers and fasten them together using a flexible craft glue.
For more realistic-looking hooks for plays or adult costumes, head to your hardware store. Many metal house fixtures are hook shaped, though you may have to shop around a bit to find one that's perfect.
Whatever you use as a hook, make sure that the straight end of the hook is long enough to act as a handle to be gripped inside a closed fist. You can extend this piece using any glued-on scrap of material, such as cardboard.
In order to cover the hand, fashion a simple sleeve large enough to cover the person's balled-up fist. You can make this from fabric, but the best look will come from a stiff material that you can spray paint to look metallic along with the hook itself. A roll of cardboard can work well enough, but a better option is if you can find a plastic bottle or jar with a round bottom of the right size.
Another option is to make a half-circle base for the hook with the handle protruding from the bottom to be concealed under the costume sleeve. Cut a Styrofoam craft ball in half and attach the hook on the rounded side. Attach the handle to the flat underside, then spray paint the whole apparatus.