San Francisco International Airport is the closest airport to San Francisco and is the recommended airport to fly into. Depending on traffic, it should take 20-30 minutes to drive from this airport to downtown San Francisco. You can also take BART, there are signs throughout the airport directing you to the BART train which is connected to the airport.

Oakland International Airport is another option to fly into. It is about a 40 minute Uber drive from the airport to downtown San Francisco, depending on traffic. You can also take the Bay Area Rapid Transit (BART) train from Oakland to downtown San Francisco but I believe you have to take a shuttle from the airport to get to the BART station.