Simple solutions:
Cut down on amount of units (your military advisor will tell you how many you can maintain without increased cost), increase tax rate
Watch out for corruption - it takes away gold
Build things like markets and banks
Don't build more city improvements than your cities can support. Buildings cost money to maintain, and citizens are your main for of tax.
Build roads, and cities by rivers and the sea because they experience much more trade. Trade = gold.
Attack barbarians. Destroying barbarian villages gains you 25 gold.
Beat on other civs and deman gold from them.