You can fly into either San Francisco International Airport (SFO) or San Jose International Airport (SJC). Palo Alto is approximately 20 miles south of SFO and 20 miles north of SJC.
Once you arrive at the airport, you have several options for ground transportation. Car rentals and door-to-door shuttles are widely available. If you prefer public transit, the BART train takes you into the heart of San Francisco and the Caltrain takes you to downtown Palo Alto.