1. The most important thing about buying a new
bike is to make sure it fits. The only way you'll know if the bike is right
for you is to size up the bike and make sure that the bike's geometry matches
your body's geometry. Ask questions and do some research.
2. If possible, try to find a shop that will
let you demo the bike on real dirt. Five minutes in a parking lot won't cut
it. You wouldn't buy a car without a real world test drive, and a bike should
be no different.
3. Don't belive the hype. Just because
your favorite rider or best friend rides a certain bike, that doesn't mean
that's the best one for you. Have an open mind and be realistic about your
needs and ability.
Shorter stems are better going down and longer stems better going up. The shorter the stem the 'twitchier' and more responsive the feel will be.
If your stem is too short then climbing steep trails will be more difficult. The front wheel will lift off the ground easily and the bike will be 'jerky' not holding a line that easily.
The stem angle will effect the height of your handlebars. Racing type XC bikes generally have lower front end positions and freeride/dh etc a higher one.
You will have to experiment to see what you like in both cases of angle and length.
Good idea in the experimentation phase to either find a bike store that will let you try a few different stems before buying one or if this is not possible buy something cheap so that if it is not right for you it won't be such a waste of money.
Not much for a man to ask I dare say.. the simple maturity to ensure a limitless supply of clean socks.