If you are planning to stay in San Francisco at a hotel - DO NOT RENT A CAR AT THE AIRPORT.
You will have to pay some pretty expensive fees to park your car - either via the hotel's parking space or if they don't have any, at a local parking garage. If your hotel does have parking, check to be sure that you have "in & out" privleges. If your hotel does not have parking space available, you will need to find a parking garage and rarely do you get "in & out" access. That means you will be paying for each period of time you are parked there which in some cases will add up to more than a full day of parking at the garage.
Rent your car once you get into San Francisco. There are many places to do that an most are close to hotels. Rent just for the day you may need to use it.
Getting around San Francisco is much easier to do via Cable Car, Muni or taking Lyft or Uber. Parking a car is difficult and expensive in San Francisco and more like an anchor around your neck as far as ease of movement from one place to the next. Lyft and Uber are really easy to use and pretty much immediate in response with the exception sometimes of rush hour or during a large city-wide event - in that case it can be a bit more time you might need to wait - however, not typically that inconvenient.
If you don't have luggage that is too unwieldy to manage or you are young, agile and muscular, BART is the quickest way to arrive in San Francisco. You may want to check which stop is closes to your hotel, or if BART exits are close to your hotel.
If your luggage is too much to deal with or your hotel is not near a BART station, you will be best with either Uber or Lyft or an Airporter Bus if they can either get you directly to your hotel or very near to it.
You can check on our Transportation page for a list of Airporter Buses and how they work and what they cost.
Lyft and Uber are the least expensive cabs coming from the airport, however you will need to download their app and sign up for an account. Both of these are very convenient and show you how far away your selected driver is from you and you can also communicate with yoru driver via the app. Payment is made through a credit card you link to your account - also very convenient.
Regular taxis are fine, but currently much more expensive - check the price and compare before just jumping into one.