Make sure every worked tile has a road built on it. Build marketplaces, banks and stock exchanges. Do not build unnecessary improvements (eg. do you really need a barracks in every city).
The form of government effects money generation. Democracy and Republic give you a commerce bonus of one gold for every square that generates any commerce.
Later in the game (when the AI has plenty of money) you may make a lot of money selling luxuries, resources and techs to the AI. Early in the game you don't make very much on sales.
There is no great "money making strategy". No matter how much you make you can spend it all. I try to keep on the tech whilst leaving my sliders so I accumulate a bit of gold per turn (enough to save up for unit upgrades and some rush building).