Hello everybody!
I am 27 years old and I am grateful to live in a city so dense that I can easily commute across town on bike in less time it takes to take the bus or train. I figure I am killing many birds with one stone by deciding to commute via bicycle: I will be reducing traffic congestion for other drivers and buses, I will be reducing potholes in the road therefore saving tax money for fellow citizens, I will be saving money on gas and maintenance on my car, and I will be getting into great shape and finally able to get rid of this spare tire.

I drive a Volkswagen GTI, and filling it up for $40 a week coupled with this nasty oil spill wrecking our beautiful coast lines is making me rethink about my gasoline consumption. I currently have an old crusty Diamond Back mountain bike that I've ridden off an on since middle school that I use to commute to and fro work occasionally, but I am looking for something I can utilize for every day or almost every day (days it is not raining) commuting for work and pleasure in the city. My work commute varies and can be anywhere from 10 miles to 20 miles a day in San Francisco, so there are some inclines to take into consideration.
I am looking to spend around $300.
I am excited about my new journey into serious biking so please point me in the right direction.
Cheers and happy Sunday!