SFO-San Francisco International Airport, a major airport hub, is actually located in South San Francisco. This facility has been greatly expanded and is very confusing to get to from the freeway (and the facility itself is also confusing).
Upon arriving to the airport, there's AirTrain connecting BART with the different terminal facilities and garages.
There are currently four terminals: an international terminal, and terminals 1, 2, and 3. BART stops at the airport adjacent to the International Terminal. If you're going international and arrive on BART, reaching your terminal is easy walking. To reach the other terminals, go to AirTrain to connect to the other terminal locations. There are airport lounges available for a fee with airlines or through independent Priorty Pass. If you need to freshen up with a shower, go to SFO Hairport located in the International Terminal, near the G gate boarding area.
TSA-Transportation Security Administration will be screening people checking for explosives; so allow sufficient time, and be calm and patient. We'll all need some patience going through this. It's best to have just one or two carry-ons for all your luggage--travel light. Leave the piano and kitchen sink at home.
Ground Transportation has BART, SamTrans, CalTrain, shuttle vans, taxis, limousine, charter service, and car rentals.
SamTrans fares 292 will get into downtown San Francisco for less than taking BART, adult $1.50, youth up to 17 $1, senior/disabled $0.75.
Airport parking is available but gets expensive quickly and not recommended.
Hotels.com is a good lodging resource.
San Francisco Visitors Center provides information for the visitor.
511.org provides transportation information.