When you live off the land, the most important thing to learn is how to light a fire. It's not just a matter of lighting a match; you must learn how to start a fire using friction and flint. You should also learn how to avoid burning yourself with the flames.
You'll need to know how to build shelter from natural materials, such as trees, grasses, leaves, etc. For warmth at night you will need to learn how to best use these materials. And finally, you'll need to know how much water you need to survive.
Other Survival Skills
Although they can help you survive, they are not as essential as knowing how to light an open fire. You can eat many kinds of animals and plants, but you won't be capable of cooking them if you don’t know how to start a fire.
You will also need to know where and how to find food, including edible animals. This is important because you could be starving or becoming sick if you don’t know.