Whether you're a college frat-boy, bartender, or just a well-prepared individual, tapping a keg is an art that must be appreciated to the fullest. While it is essentially a simple process, there are several errata that must be taken into consideration before the festivities can begin. I will assume that you are going to be dealing with a half-keg of beer, although half-kegs over other beverages, most notably root beer, are not uncommon.
Step 1: Pick up the keg
Hop in your car and drive to your distributor or liquor store. Be sure to call ahead of time to make sure that they A) infact sell kegs, and B) have the type of beer that you desire. Once there, pop open the trunk and either nestle the keg in a makeshift cradle in the center of your trunk, use stoppers, or tie it down so that it does not slide around too much. The keg is essentially a giant beer can, if shook too much, it won't be good. The keg will not explode, but you'll end up with too much foam, and not enough beer.
Step 2: The Set Up
After safely arriving home/at the party site, lift the keg out of your trunk and carry it to the serving site. A keg is very heavy, around 120 pounds. Do not drop on your foot and lift with your legs as to not hurt your back. Having a buddy help with keg movement is not a bad idea.
If you don't have a kegerator, a refridgerator designed for a keg, you'll have to make due with a bootleg version. I recommend a large garbage can, preferably plastic, full of ice. Be sure to clean the trashcan out with lysol or another cleaning agent. You can wash it in the shower, or outside with a hose.
At the very minimum, the level of ice should cover up the bottom half of the keg. The bottom of the keg should sit on a small layer of ice. I recommend putting in two pound bags of ice, and then putting in the keg, then adding more ice. Hopefully, you have plenty of time to let the keg sit and settle for a while before the party. Ideally, it should sit for atleast a few hours, but it all depends on how shaken the keg was during transportation. The more it was bounced around, the longer it will need to sit.
Step 3: TAP!
So the keg's on ice, and all that needs to happen is the actually tapping of the keg. Do you have the right tap for your keg? There are two different types of seals that I know of, one's for domestic(American) brews, such as Budweiser, MGD, and the Beast, and different ones for Heineken, etc. A domestic tap will not work in an import. You can rent taps at the distributors or the liquor store, be sure you have the right one! If you don't have the right tap, you'll have to go back and get the proper one.
Now, to tap with the proper tap. Both domestic and import taps attach to the keg in the same manner. Tapping a keg is a two step motion. First, place the tap over the seal on top of the keg, then push down on the tap. This will break the seal on the keg. Now, while still applying a downward force, twist the tap clockwise as far as you can. This will lock the tap into place. If the keg has not been given significant time to rest, you might get a beer shower while tapping the keg, so be sure to let it rest as long as you can.
Step 4: Serve and Enjoy
The keg is tapped and it's time to pour the beer! The first couple of glasses poured from the keg will be foamy, so don't be discouraged when it happens. There should be no need to pump the keg right away, wait until the flow of beer slackens before pumping. To pour a beer successfully, you'll need to tilt the glass to about a 45 degree angle. Pour onto the bottom side of the glass and let the beer roll down it, this is to prevent a large amount of head from forming. Always serve ladies first. Try to avoid pouring any beer into the trashcan, even if it's foamy. Pour it into a cup, and then the sink. The trashcan will smell really rank at the end of the night if there's a lot of beer sitting in it.
To avoid lines, fill up pitchers with beer and use those to serve, or just have someone who's worked a keg before pump and serve. Ideally, there should be no line, but we all know that will never happen. A half-keg contains about 200 cups of beer, a quarter keg about a hundred. Be sure to return both the keg and the tap after use, so you can get your deposits back. Happy partying.
This node is written off of personal experience, as I was a college Fratboy, and damn proud of it.