The Bay Area Rapid Transit system has two stops in Berkeley. The BART is a quick and convenient way to get to San Francisco. Other interesting locations are El Ceritto Plaza and El Cerrito del Norte (Target, Barnes and Noble, Trader Joe's, etc...)AC Transit
The AC transit bus system is free for Cal students, make sure to get your Class Pass and use the bus!
In particular, the F is a nice, free-to-student alternative to taking BART to San Francisco. Major routes include the 18 (up and down Shattuck) the 1 (up and down Telegraph), and the 51 (up and down University and up and down College -- though it's known to run woefully slowly at peak hours!).
are ok, 511.org
are good to know about too. 511.org allows you to plan public transport trips anywhere in the bay area. nextbus.com works with AC transit to provide you with the GPS-based positions of the buses. You can get real-time updates on when the next bus will arrive at nextbus.com.
Bikes are a great way to get around Berkeley. However, bike theft is huge around here. Make sure you have a U-lock that is hefty and that you either remove your front wheel or get a cable to lock your wheels to your bike as well! Note that you can take your bike on BART but not onto the first car.