There are quite a number of blogs I've linked to in the past that cover various aspects of WoW economics, including various tips on how to get rich quick in that virtual world. In this post I'll just take a top level view, and sort the various ways of earning gold into three categories, listing the advantages and disadvantages of the various methods.
The first category is "flipping". That is you buy something from the auction house, and sell it on the same auction house later for a higher price. The big advantage of that method is that it doesn't require any level or character skill, you can do it with a level 1 bank alt. The only thing you need is starting capital. The disadvantage is that you need to correctly identify which items are cheap and could be resold for a higher price. There are addons like Auctioneer to help you with that, but you better know what you are doing, or you might lose your capital instead of making money. Especially in Wrath of the Lich King prices for everything have been pretty much dropping since day 1 of the expansion, so something posted at a price that might have looked cheap last month might well be overpriced today. Nevertheless there are always people wanting to get rid of something fast, and posting it cheap. And depending on how much starting capital you have, and are willing to risk, you can make a rather large fortune in a short time. Or lose one. Your success might also depend on how many other people are trying exactly the same thing at the same time. A good method for experts, but maybe not the thing you'd want to start with if you never followed AH prices.
The second category is "crafting". Again you buy something from the auction house, but then you transform it into something else before selling it at a profit. Again you need some knowledge or addon to track AH prices, because not every transformation creates value. Some players are willing to lose money on crafting something, just to earn the skill point, so it is totally possible that lets say frostweave bags sell for less than the cost of the frostweave cloth and infinite dust needed to make them. How profitable a transformation is depends often on how rare the recipe is you are doing. Recipes with a long cooldown or recipes that are difficult to get hold of generally produce better profits, because there are less people doing the same thing. On the high end of this category is crafting epics, which usually takes very expensive materials and results in very expensive epics. Risky stuff, because usually there are quite a lot of crafters that can make those epics, but some shy away due to the high initial capital investment. If you're selling the only crafted epic of that type, profits can be quite nice, but if there are ten of them on the AH, you might be forced to either wait a long time or lose money on a quick sale. A disadvantage of making money by crafting is that you need a high-level character with a high skill in that craft to maximize profits. Getting your level high might be something you already did anyway, but getting your crafting skill up usually costs you a lot of time and/or money.
The third category is "farming". This is the most basic form of making gold in WoW, you go out into the world and gather stuff. That could be done by gathering ore or herbs with a gathering skill, or by killing specific monster for specific loot. Cloth used to be something easily farmed and sold, but nowadays you get a much reduced result on the highest level cloth if you aren't a tailor yourself. But then you can still farm things like eternals, from killing various elementals (or gather them with engineering). Again you need a high level (and possibly high gathering skill) for maximum profit. You *can* farm low-level materials and get more money for them nowadays than 4 years ago, but the materials you can only gather at high level are usually more profitable. Farming is not necessarily the fastest way to riches, but usually it is the safest. You can't lose money unless you try to farm monsters that repeatedly kill you. So whatever you gather will be worth something, although of course the more people who farm the same stuff as you, the lower your profit. Drops in prices mean you earn less, not that you lose some investment. And if prices for items you can farm drop far too low, you can move to the special sub-category of farming daily quests, which pretty much have a fixed income for close to fixed amount of time spent.
Which of the three methods you prefer is totally up to your personal preferences. Do whatever works for you, some people can be rather judgmental on the issue. You'll hear everything from "farming is for idiots" to "flipping is immoral". But in the end it's all just time spent in a video game resulting in a virtual reward. Just go for maximum fun.