As with most large cities, SFO is a ways out of town. Getting into town is not hard. There ate taxis, buses, and the local subway called BART. (Bay Area Rapid Transit) there is a BART station in the airport, just take the skytrains to it.
BART is, sadly, not as cheap as other big city systems but it is fairly quick. My trip downtown from the airport cost $8.10.
When purchasing a ticket, you must first know how much the trip will cost, then purchase a ticket for that amount. The nice thing is that you can use cash or credit card at the machine.
The trip into town took 25 minutes.