From San Francisco, take 580 to CA-120 (towards Yosemite). Once in Yosemite NP, proceed for several miles until Crane Flat. Turn left on CA-120 to go to Tuolumne Meadow. Going straight at that intersection will take you to Yosemite Valley.
This wonderful part of Yosemite NP is only open during summer months due to seasonal road closures.