Trade is usually the most important way to get money flowing in, especially in the early years of your city, when taxes won't bring in a lot of money yet. In Emperor, you can establish trade routes yourself, unlike the previous citybuilders: there you just got the trade routes on a silver platter at the start of the mission.
To establish trade, you have to send an emissary to a city to request trade. If the city's favor towards you is high enough, they'll agree to trade and you may place a trading station or trading quay to trade with them. Read the City Favor page for more information regarding the attitude of cities towards you.
Trade Stations and Trading Quays
To buy and sell goods, place the trading station/quay and right-click it, there you can set the goods you want to sell and/or buy, and how much you want to sell/buy per trade caravan/boat that comes to the trade building. So a setting of selling '8' doesn't mean you will only sell 8 loads per year, but a maximum of 8 loads for every caravan/boat (if there are enough goods in the trade building), up to the yearly limit, which is either 12, 24 or 36 loads.
When the trading station/quay is set up properly, and you have goods to sell in it, or have set it to buy something, traders will arrive soon. For trading stations, those are trade caravans consisting of a trader and three animals behind him to carry the goods. Those animals can be water buffalos (from a city with a humid climate), donkeys (temperate climat) or camels (arid climate). For a trading quay, you'll soon see a trade junk coming up the river and sailing to the trading quay.
The benefits of trade
Why should you establish those trade routes? First of all: to make money. Without cash, you can't do a lot in Emperor. Selling goods you can produce, especially the more expensive goods (silk, bronzeware, carved jade), will bring in a lot. Second: you can import goods which you can't produce yourself, thus you can evolve your housing to a higher level, complete that monument with imported stone, etc. Go to the standard prices page to see how much the price of a good usually is. The prices may vary between campaigns.
At the start of the mission, you'll want to decide with which goods you're going to make your money; my cities usually sell a few items of a good in the first year already and get profitable in the second or third year. Importing uncut jade, carving it, and selling the carved jade is a lucrative industry: buying it costs 95 cash, and it can be sold for 230 cash, leaving you with a profit of 135 per sold item (though this is without considering labor costs for the trading posts and carver's studio).
You can decide yourself at which price you sell the goods. If you want, you can sell say, silk, to city A for 150 cash and to city B for 250 cash. This is set at the trading station/quay. When the price turns red, however, it means that the city won't buy the goods for that (high) price: you'll have to put the price down to sell it. Sometimes, you can jack up the price to twice the Imperial Standard Price, but the city you're selling goods to will a) charge higher prices for the items you can import from them, and b) grow angry with you.
This way, it's also possible to buy goods from one city, and resell them to another city for a higher price. Usually this isn't worth the hassle, but if you're very tight on cash this can just bring it the little extra you need.
At the Commerce full report, you can set the price you are willing to pay for each good in the 'Max Price Pay' column. By default this is set to 10% more than the imperial standard price. At this report, you can also change how much you ask for the goods. If you change these values, it will change in every trading station/quay that can sell that particular good. In the last column, you can see if there are any buyers/sellers for the goods. "--" means that there are no buyers/sellers; "Yes" means that there are buyers/sellers for that good and that you are already selling to one or more of them; "No" means that you can sell/buy the good but aren't currently doing it. Clicking on Yes/No pops up a list of buyers and sellers for the good.
If you have a working money printer or mint in the city, you will get a little extra (about 4% of the total price) for every good you sell, although the normal price is still listed in the trading station.
All in all, trade is usually a great way to make money and should be started as early as possible.