First I want to point out the previous reviewer is wrong--you DO NOT need a reservation for this tour. It does have limited days and hours because it is solely manned by volunteer staff, so check with your lodging facility or with Big Sur National Parks and they'll be able to help you with seeing the lighthouse.
When you arrive, get there at least 20-30 min early so you are guaranteed a spot for the tour. They only take 40 people on the tour. Park along the side of the road to the entrance. The line will begin forming at the gate so make sure you are facing south on hwy 1.
The tour begins by walking the roadway up the side of the hill. It is a .88 mile round trip hike on a completely paved course. If you aren't the hiker type, the hike itself is easy as the tour guides stop several times along the way to tell about the history and point out various wildlife and plants.
At the top of the hill you'll be able to go inside the lighthouse, but don't get to stay too long, but definitely long enough for pictures and asking additional questions if desired.
During the entire tour, it is full of history and interesting information, but I had no proior knowledge of the island or lighthouse before attending. I just enjoy lighthouses!
At the very top of the island you pay the $12 USD for an adult ticket for the tour and you can pick up a souvineer, or after you pay, you can watch various short films about the history of the island and lighthouse.
Overall, I would highly recommend this tour because it is fun, interesting and gets you out of the woods, river, car or off the beach for 3 hours.
And just by going, you help our national parks :)
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.