If you live in SF and work at the Embarcadero, just skip BART and ride the whole way.
You could ride a fixed gear with brakes from almost anywhere in SF without hitting major hills.
If you are new to riding, a geared bike would be an easier start.
My recommendation (but ultimately depends on how much you can spend), an internally geared bike with wide (700x28/32) tires and a rack in front.
Internally geared so don't have external mechanicals to gunk up or get damaged.
Wide tires for comfort and easier time rolling over urban street "features".
Front rack because a load in front is easier to handle. (Moot if you're going to use a backpack.)