The National Park Service runs a ferry service from Fisherman's Wharf to the island. I believe that's the only way to get there. While on the island, rangers lead tours. There is also a self guided audio tour that you can do. Headsets are provided when you arrive at the island.
Its best to book this way in advance as the ferrys often sell out, especially in the summer. Visit the park service website, www.nps.gov and find the Alcatraz page. From there, I believe you can find the information to reserve a spot on the ferry.
Hope that helps!
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