Ok, this is something I wish I had read when I started out playing. It is an easy to follow guide on how to start out making gold off the auction house beginning at level 6. A lot of the methods here are posted in other threads I am sure, but this is a compilation of the methods that have worked for me, time and time again. I can take any level 6 character and have him ready to buy his own mounts (yes, standard, epic, flying, epic flying) by level 40 without doing much more than an extra half an hour a day. Granted it takes time to get it going, but if you stick to it, it will work.
First, get to level 6. Then go to whatever major city is yours. Locale is not an issue at all. Take two gathering professions, one of which is skinning. I hate herbalism so I never mess with it, however enchanting (for the disenchants) is very nice. If you take enchanting, RESIST THE URGE TO BUY ANY RECIPES! I know it is hard to pass up all the pretty new things, but wait until you are rolling in gold, then do it. Right now you are solely focused on gathering.
Now, if you have not already done so, download Auctioneer. I prefer classic, but either one is fine. This is your GUIDE. This should not be the only tool you use to price with because it is entirely possible that it will be fooled. You will use it to help you gauge if something is cheap or overpriced, but the actually pricing you should do. Now, go run an auction house scan with it before you leave the major city. Then return to whatever you were doing, most likely questing.
While questing, you will see your bags begin to fill up. Return often to whatever small city or town is nearby and sell the trash (i.e. greys). Do not sell things just because you wouldn't use them. These are going to help you get your starting capital. Also, at this low of a level, whites do sell decently on medium-high servers so keep them. When your bags are full with auction house materials, return to your major city and run another auction house scan. After it is done, see what you have that is either able to undercut or has no competition. Most likely you do not have enough money to buy out competitors so we will get to what to do with the stuff that is listed as "can't beat lowest price" in a second. All those that are undercut or no competition, put them up right away. Look at the price Auctioneer lists for you as well as the prices of other similar auctions and see if the price makes sense. If it seems to low, raise it. Too high, lower it. Use common sense, basically.
Now for the rest of the stuff in your bags. These items all have competition that is lower than what Auctioneer thinks it should be. Look at the other auctions and decide one of two things: Sell it below the other guys or hold onto it until these other auctions expire. This is really a case by case basis because some items, like cloth, will almost never sell out, whereas blues and epics will. Use your best judgement and don't be afraid to make mistakes because you are going to lose money every once in awhile. The key is to learn not to do whatever it is that made you lose money again.
You should continue this method until you reach about 20g. With some diligence this can be done in a few short days with out taking you away from leveling. Also, since I didn't mention it before: DO NOT BUY YOUR EQUIPMENT OFF THE AUCTION HOUSE! You can get decent equipment yourself just by killing mobs. You use the Auction House as your tool so do not let other people make you their tool.
Once you hit 20g you are going to begin scanning the Auction House for great buys as well as continuing to post your own items. Use Auctioneers Search Auctions tab and set the options to Buyouts, 30% profit, 50s profit. Yes, this might seem low to you now, but if you try and buy that 10g item that is supposed to make 30g and you lose, you have lost half of your capital. Start small and let it snowball. Trust me, it works. Now look through the auctions and try to stick to things you know sell. If you have been selling a ton of linen cloth and you see some that is way underpriced, buy it. Stay away from recipes. Period! I don't care what anyone tells you about buying them from hard to find vendors and reselling them for oodles of gold. It is all luck with recipes because they are not reusable, like cloth or herbs. Once someone gets the recipe, they won't need to buy it ever again. So you are pretty much hoping some schmo will need your Enchant Bracers: Minor Strength right now and be willing to pay 3g for it because he isn't smart enough to walk next door and buy it from the vendor for 15s or whatever. Also, if you don't know what something is for or if it is a good item, don't buy it. I lost my shirt trying to buy up all the Darkmoon Faire cards because I thought they were neat and they would sell well. Sure enough, the Furies and the Lunacy ones did but I still have 15 warlords cards, 2 decks of Portals and I don't even know how many Beasts. If you don't know, don't buy.
Now, as you progress in levels and in gold, your next waypoint is 50g. When you hit this change the 50s option to 1g and continue the process. This is normally around level 15-17 for me. Then comes 100g; change it to 2g. At 200g change it 5g and keep it there. Now you should be buying blues mainly and high level enchanting items. The other reason I like enchanting items is there is no deposit for auctioning them so I don't lose as much if it doesn't sell and can just relist it.
This simple method will easily carry you on to purchase whatever you want throughout the game. The key is to be persistent and not to get discouraged by loses. I have lost all of my gold investing in epic items no one wanted, only to make it back selling disenchanted greens that I bought for 10s buyouts (which is another great idea- Scan the auctions for Uncommons and buyout anything 10s or less. Strange dust normally sells for about a gold a stack and you can get anywhere for 1-4 per disenchant at the worst). Also, don't try and sell big items, like your Shadowfang on any day other Friday. Put it up on Friday and wait for the twinks to scarf it up over the weekend. Use the weekends as your big sell days and the weekdays to pick up cheap items.
Final Auction House tip- Tuesday maintenance. Log on about 15 minutes before the server shuts down for the eight hour maintenance. Go through the auction house and look for any uncommons or better that have really low bid prices. I have picked up things like Diamond Hammer for 12s just because I got the last bid in and the auction ended during the maintenance time. It might sound cheap, but you should be as miserly as possible. Well, good luck and if you have anything further to add, I am open to suggestions. I did try to keep this general and avoid specific items or grinding spots on purpose because I want this to be an all-purpose guide and allow people to insert their own spots and items as befitting to their servers. Thanks.