Handlebars - Mountain Bike Zen
Adjsuting Bars and Stem
The adjustment of bars and stem is usually very straight forward. Loosen them up and position them and then tighten them up again.
Most modern bars will have markings on to help you centre them on the stem. When tightening up tighten each bolt a little and move between them rather than doing one up as tight as possible and then moving on to the next one. This helps keep an even pressure over the grip of the stem on the bars.
If changing out the stem you need to check it will fit your steerer tube and your handlebars. A longer stem can help gain a more aggressive riding position, a shorter stem will provide more control over pulling the wheel up over obstacles etc. and more direct control over steering although steering will be slightly harder.
If using your bike on road alot you may want extra upright handlebar grips, these have little place on real off road bikes though and are only going to provide an extra object you can severely injury yourself on.