Airports:
San Diego (SAN) and Los Angeles (LAX).
Major airlines which fly to/from San Diego are here.
From Europe, consider LTU Airlines (Düsseldorf-SAN ~$400.00, or Frankfurt-LAX ~$275.00)

From LAX, you can fly, rent a car, take a bus, or take the Coaster train to San Diego (Don't forget to get off in SD, otherwise you'll wind up at the US/Mexico border!)

Tourist Information:
Lonely Planet San Diego