The answer to how do you make gold in World of Warcraft depends on your class and your professions.
First off, certain classes do better at "farming" than others. Rogues are generally considered the best farmers because they don't use mana so there is little to no down time between fights. Mages are great because of AOE but they need to stop after nearly every fight to regain their mana. Arms/Fury warriors can make excellent farmers as well.
The best way to make money is warcraft is to get the rare recipies/formulas/patterns/pla... and farm the materials to make them or have others farm them for you and set up a system that has them CODing the mats to you on a regular basis. The key to using a profession to make money is to have the crafting rare and desirable items people actually want and then getting the materials below AH costs.
You can also make money by spending hours and hours gathering the hard to get materials. Certain herbs sell for 15 - 20 gold per stack on the average server. Thorium and arcanite crystals all sell well. You just have to get a familiar route down in a few zones and you'll be able to churn out a decent profit.
The last option and I believe this is the one you are actually referring to when you say "farm" is to find epic world drops, or highly desired blues and sell them on the AH. This can be frustrating, much like playing the lottery. You will spend days on end not finding anything worth while, then you'll finally find an epic drop that you can sell for 500 - 800 gold. For this you need to kill creatures 55+ that are non-elite. EPL Fungal Vale is a very popular place for farming. Azshara is another. Just about any zone that has 55+ mobs will work as world drops are random drops based on creature level.
How much time DO you have? Are you the type of person who can go days without seeing anything worth while as you wait for that rare epic drop? If so, by all means go to EPL or some other comparable zone and get to work. If your time is limited, work the AH and sell your rare craftables. Often it isn't the items you can sell for 100 - 400 gold that are going to make you money but it is selling a steady stream of 2 - 10 gold items. If you have a craftable item that is hot, you might be able to make 50 - 100 gold a week just making a few a day and selling them. Easy, steady gold. It all depends on how much time you are willing to invest.