The route is simple: I-5 runs between the two cities - but driving isn't the only way. If you can't (or don't want to) drive, you have other options.
San Diego - Disneyland by ShuttleA once-daily shuttle runs between San Diego and Anaheim, not suitable for a day trip, but fine if you're going for a day or two. Reserve your shuttle online.
San Diego - Disneyland by TrainThe closest Amtrak stop to Disneyland is about two miles away. The Anaheim station at 2150 E Katella Avenue is served by the Pacific Surfliner route which will take you to Old Town or downtown San Diego. Get fares and schedules at the Amtrak website.
If you're arriving at San Diego Airport and going straight to Disneyland to start your vacation, you can take the San Diego MTS Route 992 bus to Amtrak's Santa Fe Depot downtown.