That's funny -- a friends son just asked me that a week ago, so I asked a neighbor who fishes and this is our advice:
Fresh water, ocean or bay fishing? The Mercury News prints a one page "fishing report" in the sports sections -- it was Thusday last week, so get the paper tomorrow and check it out.
Also there is a good fishing store on De Anza Blvd in San Jose called "Fisherman's Warehouse" (or something like that). They will be able to tell you where to go (it varies month to month and year to year). They can also sell you a fishing licence (needed for freshwater fishing -- not sure about the other types) The store is just a little north of HWY 85, near the Mc Donalds (its between the cities of Cupertino and Saratoga).
When my 8 yr old daughter really wanted to fish (and I know nothing about fishing, and have no equipment) I brought her to the "sure catch trout ponds" right by the intersection of HWYS 85 and 101 in San Jose. I came home with 4 trout, but had to clean them myself. I thought I was smart because I thought to bring a cooler with ice in it to bring the fish back home -- it was a good choice. The fish were delicious. I've seen other people fish in that general area -- perculation ponds I think -- I'm not sure what its called.
Have fun!