If the animal is still in the shell, you need to put the shell in some water and SLOWLY bring it to near boiling to "cook" the animal. (Good eating, usually!!) Some shells are not compatible with boiling, as it may craze the nacre. Pull the animal out slowly after it is fully cooked and make sure that you get all of the liver out (the last little curly-cue part that is in the shell- dark colored).
Another method the get the animal all out is to place the shell on an ant hill. The ants will clean it out in a couple days, usually.
Once you get the animal out:
Cleaning seashells can be easy or hard depending on the condition of the shells. The best and simplest system for MOST shells (some are too fragile for this) is to soak them in diluted (1:1) bleach for 10 minutes to a few hours (depending on the degree of encrustation, etc.) then cleaning any detracious material from the surface with a pick, if necessary. Repeated bleaching could be necessary. The pariostricum (the "skin" that covers the shell) will come off in the bleach.
After the shell is clean, brush on some diluted mineral oil or baby oil.(some people use other oils). You can dilute it 1:1 with some lighter fluid or paint thinner (FLAMMABLE!!). This will bring out the color and shine of the shell.
For a more professional method, see our ultrasonic cleaning machine.