How do I make a lot of money in Civilization 4?

Actually, this is explained in the Game Concepts section in Civilopedia, but I can try to do it in fewer words here.

Your economy in Civilization IV is based on your population (in cities). When you look into the city info screen you see white circles, which symbolize the population of the city (one circle for one population unit, except the city tile itself, which does not count). Here is a screen shot:

To support your state (cities themselves, like city administrations, support your civics, buildings, troops) you need money. You can get money in several ways:

• convert a part of commerce, which is produced by the population of your civilization, to money. (the amount of commerce which is produced by a tile is indicated by golden circles (each one stands for one unit of commerce), or a purse with the dollar sign (stands for 5 (or 10 in rare cases) units of commerce) or a combination of the two). This option you tried already and still have -10 amount of money for a turn.

  On the screen shot your can see the "Ivory Bend" mark. This tile produces each turn 0 commerce, 2 food units, and 2 production units. It produces this because it has a population unit working on it (white circle) and it has two hammers (production, like golden circles-coins stand for commerce) and two bread slivers (stand for food). The tile right above the marked tile produces 3 commerce for each turn and the rightmost tile in the upper row produces nothing at all because it does not have the white circle.

  Also commerce is produced by trade routes (foreign and overseas are very fertile ones).

(I think, you've got it.)

• the second way you can get money in the game is from specialists (merchants, priests and artists produce some amount of money). On the screen shot you can see a great merchant who produces 6 units of money per turn.

• the third way, if you or another civilization have the currency technology, you can trade money with another civilization: for technologies, peace treaties, tributes you can get (or give) one single lump of money. For resources, peace treaties you can get (or give) some amount of money per turn.

• the fourth way, you can convert production in a city (all hammers being produced in this city) to money.

So as a summary I can say, that you need to produce more commerce, that is put more white circles in your cities to tiles with commerce value. And the question is then: what is the most efficient way to have more tiles with commerce?

This is very complex question and it is hard to give a complete answer for it here. There are innumerable ways to achieve this. One of the way and a rather robust one (but I think not the most efficient one) is to built more cottages (you can read about this improvement in the improvements section of Civilopedia).

My personal favorite way is to choose a financial leader (Inca's leader is my favorite one) and tend to built coastal cities from the start of a game and try to build the Colossus wonder, which gives you a huge 4 commerce units on coastal tiles at the early stages of the game. Then, when I achieve the Democracy technology, I switch to the Emancipation civic and after that I put my citizens from the coastal tiles to tiles on land, preferably grassland where the cottages are already built. And then after 35 turns of work on a cottage I get 9 units of commerce in a cottage tile (4 commerce for a town (the last stage of a cottage), 1 for the financial leader, 1 for the Printing press, 1 for a river tile, 2 for the Free Speech civic).

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As a side note, not related to the question itself, but related to new players.

You definitely need to play some time to get accustomed to the game to grasp all its concepts. As one very important advice I can give you, you need to pay more attention to national wonders: there are very useful ones: the Heroic Epic, West Point, Ironworks, the National Epic, Oxford University, Wall Street.

The idea, is that you should make some of your cities specialized. One city should be oriented towards research and money, it should be coastal (because it produces more commerce from trade and has more health, that is allows more population) and its tiles should have more cottages (not watermills or workshops) and you should try not to built anything except useful buildings in that city.

One city should be oriented towards military units production, you should have the Heroic Epic and West Point wonders in it and city's tiles should not have cottages at all, instead only those improvements, which give production: watermills, mines, workshops etc.

And so on. Hope this will help.

You could try turning your tiles into gold-producing tiles. Be conservative, though, as liberal application can lead to issues late game.