Many of us love eating fish in restaurants and are aware of its health benefits, but have no idea of how to cook it at home. Actually, the challenge is not so much cooking fish, as shopping for it! The key is finding a good, clean fish market or counter and getting to know the fishmonger, who will help you find the freshest fish available and will clean it for you if you can’t stomach doing it yourself.

LOOK HIM IN THE EYE! (the fish, not the fishmonger)
A fresh fish should look alive! Its eyes should look clear and stick out, they should not be opaque and sunken. Its gills should be wet and bright red (if they are brown or grayish, your friend was frozen); the skin, flesh and fins should be firm and bouncy, and the scales shiny.
It should smell like the ocean, but not TOO fishy. If you are buying fillets, be aware that brown edges or red streaks show age and should be avoided. You should always prefer a smaller fish over a larger one, because it will have more delicate flesh (one exception, I believe, is Turbot). I avoid fish that is sold already wrapped in plastic. If you have to buy it, at least rinse it very well and pat it dry with paper towel before cooking. However, it’s best to buy fresh fish, or even frozen. Most frozen fish nowadays is frozen fresh and can taste great if you let it thaw slowly in your fridge, rinse and dry it, and then cook it immediately.
What you should avoid at all costs is fish that was frozen but is being sold as fresh, because its texture will be unnaturally mushy.