This might be a dumb answer, but anyway ...
Citizens consume 2

(healthiness and happiness catered for). If you have a 'surplus' in your city of +1

, eventually it will grow, and possibly under some circumstances, the city will end up producing -1

, as your new citizen is eating 2

, but maybe not working a tile that puts any food back into the system (e.g. working a Desert Hill Mine, or employed as a specialist citizen).
If this is making sense so far ... you can either 'see-saw' the city's food, so that for a few turns it produces a food surplus (getting close to full before you hit starvation), and then a few turns later you rearrange your citizens so it produces a food deficit for a while, but the city never actually grow so big that it starves ...
... or you can put the City Governor on 'Avoid Growth' which will prevent the population growing out of control ...
... or if you're in Slavery or Nationhood civics you can 'whip' or 'draft' excess citizens.
... or you can just ignore it all together, which isn't 'great play', but at the low difficulty levels is tolerable.
Food resources are terribly important in Civ4, and I agree with Gwynnja that you should look for opportunities to settle cities near (*that is, within their 20 'workable tile radius' or 'Big Fat Cross') food resources such as Corn, Pigs, and Sugar.