Run it through the PA with a mic if maintaining the distinct tone or overdrive of your amp is important to you. The Shure SM57 and SM58 are the "standard" mics for miking guitar amps. Place the mic about an inch from the amp, pointing to the center of the amp speaker cone.
Use a direct connection. Some amps have line outs that sound surprisingly good, and using a direct connection will always be more convenient because you won't have to worry about sound leakage or feedback. If you prefer this method, run a XLR or guitar cable from the line out jack on your amp into one of the inputs on your PA.
Experiment with the volume of the guitar amp through the PA. In general, the best guitar sound will come directly from the amp itself, so use as much guitar amp volume as you can.