Fish cookery is simple, it really is.
The scaling, gutting and all the messy jobs are done by your fishmonger, so you should not have to worry about anything but the cooking.
Fish is forgiving; it will not go from undercooked to overcooked at a blink of an eye. It will wait in a warm place for a few minutes while you finish your sauce or vegetables; but it does not like being left under a hot grill for two long.
Fish like cod, hake and haddock, with soft large flakes is better roasted or baked, the less you mess around with it the better, it will break up if over handled.
Firmer fish like monkfish, turbot, brill, mackerel and tuna are more robust and stand up to a bit of handling and a fierce grilling.
Try and use the oven when cooking fish. I almost always start cooking fish in a pan but once it has become golden brown on one side I turn it over and pop it into a pre heated oven.
Fillets of Haddock, Cod, Pollock, Turbot, Brill and John Dory can be browned in a pan first. Heat some ground nut oil until smoking hot (a cold pan will cause the fish to stick). Put the fish in the pan, cook for 4-5 minutes, then turn it over when it is nicely coloured. Put the pan in the oven or transfer the fish to a roasting tray and finish it off in the hot oven. This method is used by almost all chefs and in my opinion it produces the best results. And it is simple once you get the knack of it.