Assuming that you've been accepted to one of UC Berkeley's programs, congratulations!
I've lived in the San Francisco Bay Area for about 4 years now (Berkeley for three years during my undergraduate days). In some respects, it resembles a Developing Country (also having lived in one myself). Nevertheless, it is a very interesting, eccentric, and eclectic place.
Where to live:
I suggest living in Berkeley for at least 6 months. There are decent neighborhoods to live in. North Berkeley is nice, especially if you're fond of good produce, good restaurants and good walking/hiking trails. College Avenue is a scenic neighborhood with a mix of professionals and college students-- just stay away from College and Piedmont. More often than not you'll come across college girls who are doing their "walk of shame" or testosterone-filled frat guys- unless that's what you're into. Student housing ranges from residential halls to single-occupancy apartments which are decent, but pricey.
How to get around:
The whole of Berkeley is bike-friendly. Lots of bike stores and a number of bike coops. A lot of bus lines service the city and the campus. Just be warned that since UC Berkeley gives out free bus passes to students for Alameda-Contra Costa County, most buses are brimming with students (and transients) most of the time. Much of Berkeley- the interesting and fun parts at least- can be seen by walking. Got a car? Leave it.
If you decide to live in the city (read: San Francisco), BART (Bay Area Rapid Transit) has a Downtown Berkeley Station which dumps you one block west of the campus on Shattuck Ave. Travel time (on good days) between the city and Berkeley is usually an hour.
Where to pump iron:
UC Berkeley has its own gym called the RSF. Student membership, last time I checked, is $10 a semester. Lots of cardio machines, basketball courts, volleyball courts, tennis, wall climbing, pool (oy, those water polo players...), track and soccer field. Lots of PE classes too. Sometimes Cal Bears athletes play side-by-side with you. You can also go down to the marina and windsurf, kayak, or row. Lots of running and hiking trails. A lot of athletes train in Berkeley for triathlons too.
There is a 24 Hour Fitness and a YMCA downtown with decent membership fees, especially if you qualify for financial help.
Where to pass time:
Lots of grass and trees. A couple of decent parks. Telegraph Avenue. The City (just don't leave the city too late in the night, or else you'll find yourself stuck in Tenderloin waiting for the 800 bus that's notorious for NOT following the schedule).
Unfortunately, there are no good gay (or straight) bars in or around Berkeley. You have to go to The City for those. There's also White Horse in North Oakland, if you like dive/tacky bars.
A lot of people underestimate and criticize Berkeley off-bat, especially the rich and cynical ones. Don't let those discourage you from trying stuff out.