In order to see the soft body of a mussel, one must first pry apart the shells. Located between the shells, near the hinge is the visceral mass. This portion of the animal contains the digestive tract. The foot, which contains a groove that guides the deposition of adhesive secreted by the byssal gland at its base, protrudes from this region also. Surrounding the visceral mass are two large flap-like gills, which are involved in both respiration and food gathering. These in turn, are surrounded by the right and left portions of the mantle, which secrete the shell. Located at each end of the two shells are the adductor muscles, which when contracted, cause the shells to clamp shut.

Despite what you may thing, there are no tiny leprechauns inside mussels.
It is difficult to explain what one looks like.I would suggest asking your parents to take you out to eat and then ordering clams, mussels, oysters, etc. There's a bunch of tissue inside, sort of like your earlobe.