Getting to and around San Francisco is easy. To get to San Francisco you can take a plane, a train, a bus, a car, or some kind of public transit.
San Francisco is served by two airports, San Francisco International (SFO) and Oakland International (OAK). There is a BART connection at SFO which will bring you into the city. There is a shuttle that will take you from OAK to the MacArthur BART station and into the city. A taxi from SFO will run you about $40. From OAK, a bit more.
BART is the Bay Area's subway/train system. It is very good for getting from San Francisco to the East Bay (Oakland, Berkeley, Richmond) and a few places in San Francisco.
From Sacramento and the Valley you can take Amtrak to the Richmond station and take BART into San Francisco or to Oakland and take a bus in. There is no direct train to SF. But there is the bus! Greyhound serves SF.
Driving is also an option. From Sacramento, it will take you a couple hours. From Los Angeles, 6 to 8 hours. From Portland, about 12 hours, and then add a few more from Seattle. May is a very nice time of the year to drive.
Once in San Francisco, you can use the aforementioned BART system. There is also the notorious MUNI system of trains and buses. They will get you around the city, but rarely run on time. They are safe but can be crowded, so watch for pickpockets! The drivers are almost always surly. And when walking across the street, stay out of their way. They stop for no man! The 22 Fillmore will get you all the way from the Marina district, through the Lower Haight, SOMA, the Mission, into Potrero Hill, about a block away from the Bottom of the Hill. A number of buses will dump you near the Hemlock Tavern.
Driving in San Francisco is okay, but parking can be a bitch (though plenty of parking around Bottom of the Hill). Cabs are all over, though snagging one on a weekend night can take a bit of time. Most of the cabbies drive like maniacs.
San Francisco is a great city to walk. It is only 7 miles by 7 miles. The trick though is avoiding big hills, which you can do. Most of the city is flat or on soft inclines. Bring comfortable shoes.