Rule of three
by Trent Hardy
This is a guideline that I've read about on a bunch of websites. It basically gives you your priorities in a survival situation. Like the name implies, it is based on threes in chronological order. It is as follows:
3 seconds to get out of immediate danger
3 minutes without air
3 hours of exposure before exposure can harm you
3 days without water
3 weeks without food
3 months before any possibility of rescue.
Notice that fire is not included in this. That's because although fire is HUGELY beneficial, it provides heat, light, cooking, and can be used as a tool for cutting large pieces of wood. It is not considered totally necessary. We're the only animals that use fire, and you can keep pretty warm if you build a sufficient shelter.
Also, remember that exposure is not just rain, snow or cold. Too much time in the sun will do just as much damage. Sunburns dehydrate you, add to any feelings of anxiety and discomfort.
I think the first thing anyone should try to learn is how to build a proper shelter.