MOLLUSCS - BUY THEM FRESH
When buying molluscs, the general rule for mussels, clams, scallops, and oysters is that their shells should be closed. This is a sign that they are still alive and haven't began to rot. The shell itself should look solid without any holes. If a shell is lightly open, try to tap on it and see if the mollusc will close it - if so, it is still alive.
Once you've purchased your bag of molluscs, you should keep them on ice and cover with a damp towel. Even better, before you cook them, you can immerse them in cold salt water (that is, about 20 g per liter/quart) for several hours, so their inner organs become filtered, and much of the sand and grit becomes removed. Do NEVER put the molluscs into plain water, because they will die if there is no salt in it (osmotic pressure is quite powerful).
To open the shell of a living mollusc, without cooking it, you can use a strong small kinfe and stick the blade in between the shells where the hinge is. Then, cut the ligament and follow through by sliding the knife along the inside of one shell. This will cut off one side of a muscle of a mussel or clam. Then remove this shell, and cut in a similar fashion, along the inside surface of the other shell to release the rest of the inner part of the mollusc.
Usually, the mollusc shell can be opened more easily by steaming them in some butter and wine (the traditional way of cooking mussels). Just put them in a hot pan, where you have built up the heat and steam, throw in the mussels/clams, preferably in a single layer, cover with lid, and cook just until their shells have opened. You will need to lift the lid to see if this actually has happened, but when doing so, do it quickly so you don't lose the steam in the pot. As soon as the shells have opened, quickly remove the mussels from the heat so they don't get rubbery. They should be tender when cooked. If any of the shells don't open, then throw these molluscs in the bin - they are not good. A good practice, since some mussels open their shells faster than others, is to remove the ones that just have been opened. Once again, do it quickly so you don't lose too much steam.
A few words should also be mentioned on when to buy the best molluscs. The thing is that you cannot eat tasty molluscs all year round, since they have their common spawning periods during which they are essentially...not good, so to speak. Atlantic mussels are best in spring, and they spawn during the summer. Mediterranean mussels are best in summer, since they spawn in winter. Mussels are also maybe the easiest molluscs to cook, since they are slightly tolerable to overcooking, but don't take this fact too much into your heart - try to cook them as they should be, that is - tender.
Oysters are usually eaten raw, although they can be cooked, but only swiftly. Live oysters can be stored in a refrigerator under a damp towel for a few days, during which the oyster will develop some extra flavor. Oysters are often bottled, and if you buy such food, look for the opacity of the liquid - it should be clear and not cloudy.
Scallops are usually not sold as whole, just the muscles, since they don't close their shells tight enough. However, living scallops are still more desirable, so that you can cut out the muscle on your own and sauté it while it's still twitching.
Squid and octopus are different, not only in their looks but also in the way they must be cooked to get them tender - first of all, if the meat is poundered (or smashed against the stones, as it's traditionally done) the tough tissue will get more tender. Then, when cooking the meat, it becomes even more tender if it's simmered for over an hour.