Oh, I think I was wrong. I just did a test start, and built my city on every type of terrain I could find. Whether it was on grasslands, plains, hills, desert, tundra it was always 2 food, 1 shield and 1 commerce. Though I was playing as the English, and since they're commercial maybe it affects it slightly.
Then I did also try to build on a resource. When I built on a cow, I had 3 food, 1 shield and 1 gold. Yet a cow on grasslands is supposed to yield 2 bonus food, so was the effect reduced somewhat?
Likewise when I built on a grape symbol, my advisor said "you've got a supply of grapes, good work". but at the same time, grapes are supposed to give +1 food and +1 commerce. But my city was 2 food, 1 shield and 2 commerce (no food bonus).
So I dunno, maybe someone else has looked at this already and knows more about it? Well default definately would be 2 food and 1 commerce. If you didnt have 2 food, your city would never grow on plains for example. The shield might be an english bonus for being commercial. I thought I remember something like that.