If you're out in the wilderness and need sustenance, Mother Nature is pretty good about providing. Options for food sources will exist, but food will not always come easy. There are times when you will have to do some work and get your hands dirty before your stomach can be filled.
Unfortunately, in addition to Mother Nature's provisions, humans also provide us with fields of litter. It is fairly common to see trash strewn about in places it should not be. While this is a dismaying sight, if you are in a survival situation, rummaging through such trash could very well save your life. Think about how often you come across discarded soda cans in your daily travels. You probably see at least one every day, as sodas are popular drinks. If you are hungry and have access to some string or twine, one of those discarded soda cans can provide you with a handy tool for catching a fish.
In your pack, you should have fishing hooks, but if you do not, hopefully you will at least have a small file and a cutting tool with the capability to cut through aluminum. With these items, you can take the pull tab off of a soda can and turn it into a fishing hook. Simply pull the tab off the can, worrying not about the tiny ring that affixes the tab to the can as you will not need this. Once the tab is pulled free, file down any jagged edges in the top ring that could cut through your string, as this is where you will tie your makeshift fishing line.
When you are done filing and satisfied that jagged edges will not be an issue, it is time to move on to creating a hook to catch your fish. Cut the bottom tab away from where it meets the center of the tab and do so at an angle. If done correctly, this will automatically create a point sharp enough to puncture the mouth of a fish. Once your cut is complete, pull it slightly away from the rest of the tab to create a larger opening for both bait and hungry fish to bite onto.
Bait can be found most anywhere, such as underneath an old, downed log or in a cool, dark place. Earthworms like moist dark areas as do other grubs. Crickets also tend to lay low beneath items that provide cover. Even a debris pile will likely yield you some bait creatures if you get to rooting through it. If all else fails, dig with your hands for worms and such that are submerged beneath the soil surface.
Once you have secured bait to your pull tab, cast it into the water. While not the most ideal means of fishing, this method is certainly better than going hungry and can result in fish for dinner if you are patient. The pull tab is also pliable so if bent can be manipulated back into a workable position allowing for re-use. You are not going to be able to pull in a huge fish, but a brim for dinner is better than nothing.