Auctioneer
Overview
To start, go to http://www.auctioneeraddon.com and download the latest version of the Auctioneer package. I'm not going to go through the individual steps of how to setup an addon, but basically you put the Auctioneer folder (and all its subfolders) in your World of Warcraft/Interface/Addons subdirectory.
Once you install, Auctioneer will add a few things to the window that you get when you speak to an auctioneer NPC. You will now have two extra tabs ("Post Auctions" and "Search"), as well as a Scan button to the Browse tab, and a UIRefresh button to the Post Auction tab. It will also add some lines to your 'tooltip', which is the info that pops up when you hover your mouse over a sellable item. If you don't see those things, check your installation. Norganna, the designer of Auctioneer, has support forums at her site where you download it.
Let me try to give a non-technical explanation of how Auctioneer works. When you run 'Scan' on the AH, Auctioneer looks at every auction listed, saving information about the items like Average Buyout price. (That's the most significant piece of info it stores.) You can check or un-check the different categories in the Browse tab, which determines whether or not Auctioneer will scan the auctions in that category.
For example, when you run the scan, Auctioneer finds 70 auctions for Netherweave Cloth, and the average buyout price is determined to be 15s per cloth (2g50s per 20-stack). Now when you mouse over Netherweave Cloth in your pack, or on the AH, your popup Tooltip will display data like Number of times seen at auction (70), and average buyout price. You can imagine how helpful it is to know what items sell for. You can know what price to list an item for (not too high that it doesn't sell, but not so low that you're missing out on some gold). But you can also search for deals...items that are listed too cheap which you can buy and resell for profit.
The Search functionality is your deal-finder. Immediately after a scan, go to the search tab and enter the filters you want to use for your search. For example, you might filter on something where you can make at least 1g profit. You run the search and find 2 stacks of Netherweave for 1g each. You can buy those, relist them for 2g45s (just under the average price), and whammo! You've made gold! Without farming, without high-end crafting recipes, and really, without the need to even be a high-level!!!