Seashells are created by snails. When snails are first born, their skins are soft, then they harden and calcify and detach from the rest of the body. They continuously grow around their own shells more and more, which is where the spiral comes from. When the snails die, the soft portion decays and the hard shell remains.
More
Some shells are also those of clams, scallops, or oysters. Depending on the location, any of these might be more numerous than the shells of snails. In any case, the process of formation is similar: A soft-bodied mollusk secretes calcium carbonate which accumulates as the shell as the animal grows, then the empty shell remains when the animal dies.