I love to eat shellfish, its one of my fav kinds of food
It is good. Shellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates
Yes my favorite is shrimp
I have herd  some kinds are found in freshwater.
Lobster is delish as well!
They are! a few species of land crabs are eaten, for example "Cardisoma guanhumi"
I have never heard of that are they good?
They are and shellfish are not a kind of fish
How are they served or cooked?
You can do it almost anyway, Many varieties of shellfish (crustaceans in particular) are actually closely related to insects and arachnids,