Buying Fish
Fish should always be as fresh as possible, so it is wise to find a fishmonger or supermarket which receives a fresh supply daily.
The supplier should be able to scale, clean and fillet the fish for you, you should ask for the trimmings if you are going to make a fish stock (after all you've paid for them).
Always buy the freshest looking fish on the day you wish to cook it - don't decide to buy a fish just because you have a recipe for it!
Listed below are a number of things you should look for when buying fish.

Whole Fish
Clear, bright, slightly bulging eyes.
Bright red gills.
Shiny, tight scales with none missing.
Moist, shiny, bright, skin and firm flesh.
An odour of the sea (Not a 'fishy' smell).

Shellfish
Molluscs should be bought alive and cooked the same day.
Shells must not be broken or cracked and tightly closed. (If the shells do not close when sharply tapped they should be rejected.)
Crabs should have rough shells and feel heavy for their size, with all limbs in tact.
Lobsters should have uncluttered shells, a slightly springy tail and feel heavy for their size, with all limbs in tact.
Prawns and shrimps should be dry, bright and firm to the touch.

Smoked Fish
Firm, dry flesh.
Glossy skin.
A good smoky aroma.

Preparing Round Fish
Cleaning
To scale the fish, hold it buy the tail and use the back of a knife to scrape towards the head.
Remove the gills, by placing the fish on its back, open the gill flap, gently pull the gills between the flap, cut and discard. Repeat on other side of fish.
Trim off the fins, using a pair of scissors.
Make a slit in the stomach and remove all of the entrails.
Rinse the whole fish thoroughly under cold running water.

Filleting
Make a cut behind the head, cutting into the flesh with clean sweeping strokes, as close to the backbone as possible, moving towards the tail.
Repeat with other fillet.

Boning - Herrings, etc.
Make a slit in the stomach and remove all of the entrails.
Place skin side up, pressing down to open out the fish.
Cut the backbone just above the tail, using the point of a knife, gently loosening the bone.
You should then be able to pull out the backbone, then lift out the side bones.

Preparing Flat Fish
Cleaning
To scale the fish, hold it buy the tail and use the back of a knife to scrape towards the head.
Trim off the fins on both side of the fish, using a pair of scissors.
Cut off the side fins, following the contour of the fish.
Cut the head off and scoop out the gills.

Filleting
Make a cut along the backbone from behind the head to the tail.
Keeping the knife flat against the bones, work the flesh away from the bones to remove the fillet.
Repeat with the other fillet, then turn the fish over and remove the other two fillets.