In Southern California (say from Los Angeles downwards in the direction of Mexico) they enjoy year-round good weather. On the coast it is likely to be warm and sunny. Further in-land it may be hot and sunny, although this is less likely in January and February.
San Francisco is to the north of Los Angeles. It is generally quite famous for foggy weather. But, in general, you can expect nice sunny weather there, too, but with less heat than further south.