Suspension on mountain bikes is a really important area, and with most suspension forks and rear shocks being air sprung, knowing exactly how to get the air into your fork and how much air should be in your fork is important to understand. The first thing to do is understand how the air goes in. Generally speaking, you'll have an air cap which unscrews and you'll have an air pump usually with a dial.
You need to know exactly how much air is going in, so you have a dial marked with psi. Screw the pump down on to the valve. That will show you how much air there is in the fork.
You can then start to put some air into your fork. How much air varies depending on how heavy you are, the type of riding you will be doing, and whether or not it's a fork or a rear shock. You can see that even adding that little bit of air in there caused the fork to extend. It's important that you have the fork with enough air in it that you're not losing travel because the fork is not opening out correctly.
Traditionally, we have recommended people set forks up using a sag measurement, which involves pushing the o-ring to the bottom of the extension, having the rider sit on the bike, then get off of the bike. This will have caused the fork to drop down through its travel a tiny bit, and leave somewhere between a quarter and a third of the travel depending on suspension preference. Rather than trying to have people run rigid sag percentages, we're trying to look at how much of the available travel you're actually using. We want people to use the total travel they've got. There's no point in having it then only using a percentage of it on a given ride. It's a good idea to take the shock pump out with you. Don't be in a hurry to rigidly follow the pressure settings advised by the manufacturer, but be looking at how much travel you are using. If you're bottoming out your fork or the rear shock, every ride, over and over again, you know you're not running enough air. If you're consistently returning home with your o-ring only halfway up the stanchion, then you realize you could probably do with a little less air to maximize the amount of travel available to you. Again, it's down to how much feel you want.
With regards to tuning, most suspension forks come with some kind of adjustment be that just rebound or compression or both. Compression control is for how fast your fork compresses when it contacts an object. The rebound controls how fast your fork re-extends back to its static length after that object has been hit. Balancing out the two so both are comfortable for you is what you are looking for. Therefore, it is a good idea to make one adjustment at a time and then go ride your bike and get a feel for the affect that adjustment has made. Then make singular adjustments, one at a time, until you get closer and closer to what you consider the perfect ride. For more information about tuning your fork check out a local bike shop or the website of the manufacturer.