Civilization 4: Choosing resources for your main city

I'm still new in Civilization 4 and had a lot of questions about playing the game. I'm finding the game to be insanely complicated so please keep that in mind while giving your answer. If you use too much game jargon I won't understand. Thanks for your help in advance.

Question: If I double-click on my starting city I come accross a map which shows some white circles. I think those circles are choosing what tiles are being used to calculate my per turn balances. But why aren't all tiles in my territory showing up? What do I do to have more options for the circles?

White circles denote fields on which the population of a city are working, thus generating food/production/trade (coins) resulting from given squares. You can rearrange the population to suit your needs, so if you would prefer more food or extra trade (coins), you can deactivate worker from active (white circle) field and assign him of her to another.

Each non-capital city starts with culture 0, thus only has access to 9 adjacent tiles. After reaching first level of culture (10 points), its borders expand and it gets full access to all tiles that are max. 2 non-diagonal squares away from the city centre. Note that every city always automatically generates resources from square composing the city centre, without assigning any population there (unless riots occur in the city).