There are 5 sources of gold:
1) the squares your city population work can give you gold -
squares with rivers, sea, roads and most resources
produce gold
2) destroying a barbarian camp and some goody huts gets
you 25gp which is alot at the start of the game.
3) you can trade with other Civs
4) you can set your city to produce Income rather than shields
(this is usually not very efficient)
5) you can sell city improvements
Certain city improvements increase gold produced - the first available are markets and the Colossus
Some civilisations are commercial these get extra gold
Different governments will give you different incomes
I suggest if you are short of cash in the early game you raid barbarian camps, build roads and found towns on river banks.
Latter in the game
- build improvements that increase wealth - each town
should have a market
- think about changing government type.
When you are connected by roads or harbours to another Civ try trading your surplus luxuries.
In the short term adjusting the tax/science/luxury rate is a solution.
Some wonders make some city improvements obsolete - these should be sold, ie. if you have build the Pyramids sell your grainaries.
Bigger towns are more economic than small towns.