If you capture a city, you begin to generate Geos based on the overall value (population) of the city.
While getting tax revenue for your cities is nice, there is one ongoing expense to be aware of - army upkeep. You have to pay to keep your armies going, based on how many troops are in them. You have to pay a certain price per day per army, plus a rate on the total number of troops. This means it's cheaper to have a few huge armies rather than a bunch of tiny ones.
Also try to improve experience of your armies instead of adding troops.
Moving armies costs oil, if you run out of oil while armies are moving it will cost you geos to keep them moving, it can be very expensive depending on the number of armies you have en route.
All troops expenses are deducted from your available geos, if they go negative, your armies will start getting smaller and smaller (no money to feed them will make them starve). However, they won't go away completely - they'll get down to 1 troop and hold. Eventually, we'll code it so that they disappear, but right now they'll stay alive.
You can deposit your geos into the world bank. It can be accessed from the "Finances" drop-down menu on the game pages.
You will receive bank interest for your money in the bank, not for the money you have in "available". The interest will be deposited into your available money. The exact interest rate (x % per day) depends on the current economic performance.
25,000 Geo Limit for Resource and Bank Interest Payouts
If you don't own a city, you won't receive any resources or bank interest any more when you saved up to 25,000 geos.
When you reach this limit, you may start spending geos on war and preferably take a few cities.