Attach your microphone to your mic stand, and make sure that it is plugged into the house PA system. Make sure that your amplifier and guitar are correctly connected, as you will be miking the guitar through the guitar amp so that it also comes out of the PA system.
Locate the speaker in the guitar amplifier. This can most easily be performed by shining a flashlight through the mesh or grill on the face of the amp. If the set up features multiple speaker cabinets, mic only one speaker per cab.
Point the microphone so that it faces the conical wall of the speaker, and not the center. When placed correctly, it should appear to be at a 45 degree angle to the center of the speaker. This angling will give you more of the natural warmth of the amp. Pointing it straight on may result in an overly treble sound.
Distance the microphone one to two inches away from the amp face. You don't want to blow out the system or have the microphone feed back by having it too close. Ultimately, the distance comes down to a matter of personal taste as suited to the PA system you will be miking the amp through. Play around with the distance, but remember not to place it too far. Above all else, you want to increase the volume of the guitar.