Living in New England we are fortunate enough to have easy access to an array of fresh food from the sea. While it is easy to pick up a fillet or two from the fish market, I think it is far more rewarding and fun to buy something "as is" and clean and prepare it yourself. That being said, here's a quick tutorial on how to clean and prepare your own squid for doing whatever you'd like with - fry it up for some calamari, make a nice italian seafood salad, or add it to your asian stir fry!
After running the squid under cold water to get rid of any debris, grab a squid and put your fingers inside of the body. Basically, stick your finger under the flap of the body right below the tentacles. From there, grab the tentacles along with the soft central bone (which looks and feels like thin plastic) along with the intestine and gently pull it out from the body cavity. You can see the thin, plastic-like bone to the right.
At the base of the body cavity you'll find two flaps that look like little rudders. Peel them away form the body so that the white flesh comes away from the skin attached to the flaps. Peel off the skin and discard it. Meanwhile, cut away the intestine (gross-looking thing attached to the head and tentacles) and throw it away. Cut off the tentacles from the head of the squid, and rub off as much of the skin as you can - something much easier said than done. If there is some left on I wouldn't really worry about it; I certainly didn't.
Depending on what you want to do with the squid, for example stuffing the squid with goodness, stop here. Otherwise you can cut the squid into rings for calamari, cut the flesh into thin strips for sashimi, or make criss-cross slits on the skin side of the body and cut into pieces for tempura, pan-frying, or grilling.
Voila! Enjoy your squid!