doesn't look like there's a way to get on at SFO, but if you are taking BART you can get one at the BART station once you get to SF.

I have found the cards useless, they don't work half the time especially if you load a cash value and not a pass.

However the clipper site does give a lot of info on how to use/load.

www.clippercard.com

Passes and ride books, High Value Discount tickets and cash are stored separately on your card and are not interchangeable once loaded onto your card.