Finding water is the first step in staying alive in the wild. Ideally, you should have a water purification pump or tablets with you. If not, iodine can be added to water to kill bacteria (nine drops for every 250 ml of water). Always avoid water in stagnant areas, where bacteria is more likely to breed. If you have to drink without purification, melt snow or choose water from a running stream.
There are plenty of poisonous plants in the wild, so choosing the right one is key. Red and white berries tend to be poisonous, so stick to blue or black berries and fruits. When in doubt, unearth plants and eat the roots. They contain more nutrients and are less likely to make you sick. Eggs of wild animals are also edible, but only if you can cook them.
Nights can get cold in the wilderness, even in the summer. Creating or finding shelter is essential to keeping your body temperature at the right level. Caves and rocky underpasses will protect you from rain and wind. You can also create your own tent-like structure by setting down branches against a rock wall and hiding under them.
You should always carry a first-aid kit with you. Don't buy a standard one at the local pharmacy but instead choose a kit made especially for the outdoors (you'll find them at sports stores). If you're injured in the wild, always stop the bleeding by applying pressure to the injury before you do anything else. Losing blood will make you weak and less likely to survive.
Learn how to start a fire. Waterproof matches are a must in any kit. Starting a fire using anything else, including a magnifying glass or a stick and stones, is possible but really difficult and requires practice. Once you get a fire going, make sure it stays on throughout the night to keep wild animals away.