How? Walk/bike around your neighborhood and pay attention to the types of trees, plants, bushes, "weeds", mushrooms etc that may be edible/have edible fruit/nuts etc (be sure to take a bag/backpack with you to collect the goods). Of course the quantity/quality you can find depends on the climate of where you live and the seasons.
Do a Google search to see if there is a "free food map" of your local area - usually information will be provided such as location of the plant, time of the year when the fruit/plant/herb etc can be harvested, whether it is on private property and so on.
To assist with identification of plants, find a book/guide on the plant-life native to or likely to be found in your area. If you are unsure if the plant is edible, it's better to be safe than sorry and give it a miss.
If the item you want to take is on someone's property, be sure to ask first.