It is a $60 taxi from SAN to the San Ysidro Border Crossing. Or you can rent a car at the airport and park it at the border for $7-10/day. Or you can rent a car and drive it into Tijuana (though I would recommend against this if you are unfamiliar with Tijuana and Mexican driving.....and the return to the US by car can be a long wait).

The Trolley will take you to the border but you'll have to pick it up downtown or in Little Italy and take it south to the SAn Ysidro Border Station. A $10 taxi ride can take you to the trolley.