Driving to Rosarito from San Diego is actually quite easy. All you will need your passport and mexican car insurance. Be sure to fill up before, since american gas is higher quality than mexican gas(though you should be fine if you do have to fill up at a local pemex). When you cross the border from the 5 south, you will pass an inspection. Right after you have passed the mexican inspection there will be signs as the road branches in four directions. There is a free way to Rosarito (libre) but the toll road is the simplest and is a much prettier drive. You will take a right towards highway one (cuota) Rosarito/ensenada. This is less than a minute past the border so try to get in the right lane soon to be prepared for it. After that you will drive along the border toward the playas (pacific ocean) and after about 3-5 minutes will need to take a right hand turn to continue toward rosarito, which should be clearly marked. At this point you just stay on the highway 1 until you reach Rosarito. The toll should be about $2.25 in Tijuana, and also in Rosarito if you don't get off before it.
Alternatively, you can take a taxi to Rosarito which is the easiest and safest for your car. You can get dropped off at the border, take the San Diego Trolley, or park on the US side of the border for around $8 a day. Right after you walk across the border, mexican taxi's will be available to take you to Rosarito. They charge between $25-$40 one way. Just let them know what hotel or destination you want. Most do not speak english but you could get lucky
