Although most buses are accessible, they are crowded and the lifts have a tendency to break down. Another option is to call a taxi. Coming later this year, there will also be an accessible trolley running up and down the Embarcadero. All stops will be accessible via raised boarding platforms. Generally speaking, San Francisco is a pedestrian friendly place, and the best way to really enjoy it is by walking or rolling around the city.
Curb cuts are plentiful, and the sidewalks are usually unobstructed.
As for the hills, well as long as you use public transportation to get from one area to the next, they shouldn't be a problem. It's pretty easy to get around in most of the tourist areas in San Francisco, with the exception of Nob Hill. Nob Hill is one of the steeper sections of town, and pretty dangerous to navigate in a wheelchair. My suggestion is to take a taxi up to the Fairmont Hotel, have dinner and then see the area on the taxi ride back. The San Francisco Convention and Visitors Bureau has a couple of excellent publications to help you find your way around town.