How do I keep the population in my cities down? My citizens keep getting angry because the city is over-populated, but I don't know how to fix it. How would I do so?
On the city screen, activate the option "Avoid city growth". This will cause you to prioritize non-food tiles if you're using automatic tile working, and your population will not go up once the food bar is full. If the food bar is full when you deselect "Avoid city growth", then your city will grow on the next turn.
Once you've done that, you can burn off your excess population by converting it into production with the Slavery civic, or by making military units with the Nationalism civic.
This may not be a problem - try a switch to monarchy, it's an underrated civic, it encourages fuller armies (plus) and allows big cities (plus).
Alternatively, use slavery and kill a bunch of the unhappy ones to get something built. This is generally a bad idea though because it will a) lower your city size -> lower production, commerce, etc. and b) snowballs if you don't let the waiting period for the unhappy citizens finish.