Trout in good condition are superb for the table. Look for well conditioned fish that are wider in the middle than across the gills. A good fish, landed quickly, will have tasty orange / pink flesh and good preparation will ensure the eating qualities are maintained. The same rules apply for other types of fish.
Iki spike the fish, or use a 'priest' to hit it on the head, and cut a gill to bleed it.
After bleeding, put the fish on ice, or in a cold chilly bin.
Gut and clean as soon as practical, wiping slime clear with a paper towel.
If you are planning to spend more time at the water's edge, prop the gut cavity open with a stick and hang the fish in a cool, airy place.