People watch the Bear Grylls type programmes and think it is a lot easier than it is. Grylls has a wealth of experience and is capable but even he iirc was caught out whilst making some of his programmes as he was making people believe he was living off the land but in between filming was tucked up in hotels.
His Wiki page is an interesting read
http://en.wikipedia.org/wiki/Bear_GryllsGeo
I've had a few little dabbles at outdoor survival. Always secure in the knowledge that I could just pack up and go home at any time.
When it comes down to it, your basic immediate needs are shelter, water and food. usually in that order. Unless the weather is very mild, you wont stay healthy for more than a night or so without shelter, and not much longer without water, perhaps a few days. Food you can do without for at least a couple of weeks.
It is quite difficult to satisfy these fundamental needs in the outdoors, even if you if you have a reasonable selection of basic equipment. Without basic equipment, it becomes extremely difficult, and without a knife, virtually impossible.
If we suppose that basic equipment includes a tarp, or at the very least a sheet of polythene, you can make a shelter of sorts. How good it is will depend on factors like where you choose to site it and how much time you can devote to its construction. That being said, even the best tarp type shelters will probably not keep you completely dry during a rainstorm, but the odd damp patch won't hurt you, just as long as you don't get soaked.
Getting out of the wind is more important than staying completely dry. The wind will suck the warmth from your body like a kid sucks juce from a carton of of Kia Ora. First priority, get out of the wind!
If you have no tarp, building a worthwhile shelter from natural materials is really difficult. If you are going to attempt it, try to make use of any kind of ready made natural feature you can find. A cave, a fallen tree, a big bolder, the lee of a hill etc, they will all make a huge difference to the effectiveness of your shelter.
Safe drinking water can be relatively simple or virtually impossible to find. If you can find a spring or a stream in the hills you will probably be OK, but most rivers and streams become contaminated once they have passed through a human settlement, or even through farmland where pesticides may have been used. Yes, you can boil out most bacteria, (assuming you can light a fire, which is a whole nother problem) but you can't boil out chemical contaminates.
Unless you are going to be out for more than a few days, food is just a luxury. If you do have to find food, it's much easier to concentrate on things like fruits, berries, birds eggs, worms, grubs and insects. The thought of rabbit stew or a fish supper might seem more appetising, but evolution has equipped prey animals with a million years worth of self-preservation instinct. They are bloody difficult to catch, and in a survival situation, your energy is your most valuable resource. Don't waste it chasing after something you probably wont catch.
Be selective about what you do attempt. Morale is hugely important in survival situations. For example, lighting a nice fire is incredibly comforting and hugely satisfying. Trying for hours and failing is exhausting and demoralising.
Nature is wonderful, bountiful and rewarding, but please don't take her for granted. She has no conscience and is totally unforgiving. She buries her mistakes.
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