Sounds like a fun trip ;).
Your (Euro) handset must have the 1900 frequency band to work with T-Mobile pre-paid in the U.S. if that's the provider you choose. I find their retail stores a little easier to deal with when I have a foreign visitor wanting pre-paid GSM service, but YMMV. Also, if you end up going off the beaten path during your stay you might also need the 850 band for roaming. AFAIK around San Francisco it's 1900. There's a coverage map here-
If your phone's quad-band you won't have to worry, obviously. I would check out gsmarena or nokia.com for the actual specs of your handset to at least figure out if it will work at all on this side of the pond. If not you can always pick up one before you leave instead of being stuck with a perhaps less desireable U.S. model that won't necessarily work back home. Where else will you be travelling in the U.S. and will you need phone service there as well?
I am in Europe right now and plan to travel to the US/california.
I have an unlocked nokia cell phone and wonder what is the best prepaid sim card to get?
Will my european cell phone work in the US?
Where to buy my sim card?
I'll be staying mainly in San Francisco - In the US for about 3 months alltogether