Head north over the Golden Gate Bridge past the viewing area on the right. Look for a sign denoting Sausalito and Marin Headlands. Take that exit before going in the rainbow tunnel (if you get there, you've gone too far). Follow the signs to the Marin Headlands (clearly marked), which are on the northwest side of the bridge.
Good luck with the weather - bring a coat because it's cold and windy. Often fog can make it difficult to get a shot.
On a clear day (fairly rare), the Golden Gate Bridge can be viewed from the Marin Headlands, with stunning vistas of both the bridge, city, and ocean.