Follow U.S. 101 North across the Golden Gate Bridge from San Francisco, or South from other parts of Marin County. Exit at Alexander Ave. If coming north, this is the second exit, after the "Vista Point". Going either way on Alexander Avenue can take you to the Marin Headlands.
If you go west, turn right before the road goes back into the freeway. You can park in a lot right next to the highway, or proceed on the road (Conzelman Road) up the hillside. Various roadside parking areas line this road. Along this road you will see breathtaking views of the Golden Gate Bridge and San Francisco. Eventually this road becomes one-way steep downhill "roller coaster" heading west, so if you pass this point you will have to return by a different route.
If you go east, turn left at the first intersection, with signs pointing to Marin Headlands. (If you enter Sausalito, you've gone too far.) This road goes through a one-lane tunnel (Bunker Road) controlled by a traffic light. After you pass through the tunnel, you will be in the Tennessee Valley area of the Marin Headlands.
If you want to search direction from your place to Marin Headlands, use the following address:
948 Fort Barry, Sausalito, CA 94965, US
There are two parallel roads into the Marin Headlands: Conzelman Road (the Coastal Route) and Bunker Road (the Tunnel Route). Conzelman Road is more scenic. Bunker Road goes through an old one-lane tunnel at one point. After Hawk Hill, Conzelman Road turns into a one-way section, which eventually intersects with Field Road. Field Road goes to the Point Bonita Lighthouse trailhead. You can also use Field Road to return to Bunker Road for the return trip (Bunker Road is the only way back after passing the one-way Conzelman Road.) McCullough Road also connects Bunker Road and Conzelman Road after the Bunker Road tunnel, but before one-way section of Conzelman Road, so you can use it to switch between the roads if one of them is blocked by construction.