Riceball said:
If you're making it from scratch you can add things to resin to give it a metallic look, I don't know exactly the names of the products but I'm sure that the more knowledgeable folks out here will.
Now if you're talking about an existing item there are a couple of way of doing it. One is painting it, but it's hard to find a good spray paint that will look like real metal. Testors makes, don't know if they still do, a really good silver enamel paint but it only comes in little bottles for painting models & miniatures; I don't know if they make a spray version of the same paint or not.
If you can't find a good spray paint then there metallizing/metallicizing kits out there that you can buy at places like Michael's and I believe that some of the people here have used them with good to excellent results.