For the love of all that is good and holy, don't drive in to San Francisco today. If you've been living under a rock, or really just don't care about sports that much, here's a protip: the Giants won the World Series in a sweep, and now all of the Bay Area wants to celebrate on Market Street. In short, the roads and ferries are going to be packed. Take a bus instead.
Golden Gate Transit has a handy guide to all things Giants and transit-related. They're adding ferries out of Larkspur and Sausalito to deal with the crowds.
Easy, you say. You'll just drive to the ferry, right? Yeah, don't do that either. The ferry parking lots are already full, and the ferries themselves are full, at least for the morning. If you have time, you can swing by the ticket kiosks at either the Larkspur Landing or Sausalito ferry terminals to see if there are any tickets left for the afternoon, but in case you don't have the time, there's one unthinkable option: the bus.
Yes, that much-maligned mode of transportation has capacity to spare. As of 8am, commuter buses were emptier than normal, so they definitely have room. You can figure out how to get into the city through 511.org, Google Maps, the GGT maps, or my own guide to the freeway below. The strip map can be printed on six standard-sized sheets of paper; just choose "Poster" on your printout. Sorry, the pocket-sized version is coming out a day late.
One other thing that might prove handy is the General Timetable to 101 (PDF), which has how to get anywhere from anywhere on Highway 101.
So don't drive to the ferry, and you probably won't get a seat even if you bike or walk there. Don't drive at all. Take bus to San Francisco instead. There's room to spare.
UPDATE: It totally escaped my mind that you could take the 40/42 bus from the San Rafael Transit Center to Richmond or El Cerrito del Norte BART stations and take those into the city instead of the direct bus lines. It'll take longer, and the buses are likely delayed this morning because of heavy traffic on 580, but it is another option. Just don't drive to the stations; their lots are probably already full, too.