What does a 1957 silver US nickel look like and what is it worth?

It wouldn't look like anything because there aren't any silver 1957 US nickels. Except for special "war nickels" made from late 1942 to 1945, all US nickels are made of the same alloy of 25% nickel and 75% copper.

War nickels were made of an alloy of 56% copper, 35% silver, and 9% manganese because nickel metal was needed for the war effort. These coins are distinguished by a large mint mark letter over the dome of Monticello on the back.