San Francisco public transport is confusing. But the clipper card is pretty easy to deal with.You can buy an actual card at one of the info desk’s at the airport. You can then top it up as you need it at any BART station downtown or Muni station – the BART machines are blue and the clipper card Muni machines are red. You can use your credit card to purchase the clipper card, as well as typing up the car did you need more value. So public transit and cable cars don’t take credit cards but you can use your credit card to purchase the clipper. Once you have a clipper card you can use it on cable cars, Bart, muni, Caltrain at Cetera.
Youth and senior cards are a separate process, which is why they’re not available at the stations or via machines. They are primarily for Residence not for visitors.