Don't get too hot. Don't get too cold. Don't get injured.

3 Minutes without Air
3 Hours without Shelter
3 Days without Water
3 Weeks without Food

Water, food, medical supplies, energy and bullets/firearms

If you're getting started with prepping then start with getting enough supplies to last a day. Then three days. Then a week. Then two weeks. Then a month. Then three months. Then six months Etc.