What You Did
You made a kind of fossil, called a cast. While a true fossil takes just the right conditions, as well as eons to be made, making an imprint of an object for yourself doesn't take much time at all.
After you tear the cup apart, the plaster should have set and you should see a mold or cast of your shell.
If you didn't press the clay down all the way to the bottom and sides, your plaster may have created a cup surrounding it, however this should not have affected the fossil itself. With a parent's permission, take a hammer and carefully break off the excess.
Try other small objects that you find outside. Try making casts with large seeds, pebbles, and other treasures.Make casts of small plastic insects or other critters, or use a plastic dinosaur to make fossilized footprints