To clean a fish, first slice along the belly of the fish, from tail to chin. This opens the chest cavity of the fish, exposing the guts. The guts can then be pulled out by hand. Try to remove the entire guts including the part inside the head of the fish.
Fish guts are completely inedible and should be thrown away immediately. You might want to rinse the fish after cleaning to remove any juices.
You can wrap the whole fish in tin foil and cook it in the oven. A mackerel might take around 25m while a trout might take around 35m.
After cleaning the fish, chop off the head. Then slice along the back of the fish, from tail to (missing) head. The cut should be along the dorsal line, deep enough to reach the spine, thus separating the fish's flesh into two halves. Then use your knife to lift the flesh away from the ribs.
Once you have two fillets, you can optionally remove the skin. Fish skin is edible but if you leave it on then you should cut away any remaining fins.
You can fry the fillets in oil or butter. Additionally, The head and bones of the fish can be used to make fish stock.