Catching your fish is the fun part, cleaning it isn't so much fun, but once you know how to do it, you can get the job over and done with faster. The sooner a fish is cleaned after catching it, the better. Fish are prone to go off quickly in the hot months especially, so get that fish cleaned and onto ice and you'll be able to enjoy eating it without worrying about the possibility of food poisoning.
First you should wash your fish under clean, running water. Don't use lake or river water if you can help it because it might not be clean. If you do, be sure to wash your fish thoroughly under clean, running water when you get home.
Having washed your fish, hold it by the tail and run the blade of your knife down under the scales with a swift movement. It's easy enough to scrape the scales off, but pay particular attention to the areas around the fins and tail. It's easy to miss a few there and nothing is worse than to get a mouth full of scales when you are eating. Unless you intend to keep the head and cook it, don't bother to scale it. Just cut it off and discard.
Some freshwater fish have a muddy taste to them. To avoid this, hold the scaled fish under running water and keep scraping the skin up and down with your knife. Then hold it by the head and scrape hard from head to tail. You'll see the 'mud' coming out of the tiny pockets where the scales were. Do this until no more mud comes out.
Cut the fish open from the gills to the vent and remove the guts with your fingers. There isn't much and it usually comes away quite easily. Run your thumbnail down the inside of the backbone to remove the blood vein there. You can use a spoon instead if you want. Some fish like mullet, have a layer of black on the inside of the flesh. Scrape this away with newspaper or with the blade of your knife. Wash the fish again and it is then ready to cook whole or fillet in any way that you want.
Never use a serrated knife, not even an electric one, when cleaning or cutting up a fish. It will shred or tear the flesh. Make sure your knife is clean and sharp and always be careful to cut away from you when using it.