San Antonio International (SAT) will be the closest airport. It is about 35 minutes from Boerne depending on the time of day and traffic.
Austin International (AUS) is going to be a bit of a drive. But if you find yourself exploring up north before or after the wedding and need another option, this is it! It will take you about an hour and 45 minutes to get to Boerne.
Renting a car would be your best option. I would book a car in advance as this is a busy time of year for the area. The rental car desks are across the skybridge at the SAT airport.
Chauffeurs on Demand is a Boerne local company that offers flat rates to and from the San Antonio airport. A one way airport pickup is $85 per person. Call Vin at 210-887-5866 and tell him you are traveling to Boerne for the Teichhammond-Matula Wedding. The drop off point is at the Fairfield Inn & Suites in Boerne.
If you type St Peter the Apostle Catholic Church into your GPS, it will probably tell you to turn on Kronkosky Street off of Main Street. That will work just fine, just follow the road around to the back, or take your first right and follow it to the back of the church where the large parking area is. The more direct route is to turn onto Hosack Street.
If you have a handicap placard, you will need to turn onto Kronkosky Street. The handicap parking is the second right once you turn.