There are lots of ways you can make money at any level.
If you are 70, yes, of course, do your daily quests. You will earn an average of 250g from quests alone (not including drops off mobs) if you do all 25 dailies.
If you are not yet 70, rely on your gathering professions. I hope you have some! Skinning is easy to level because you skin the mobs you kill. You can gather without thinking about it. Leather doesn't always sell very well, but you will make up in volume what you lose in price per stack. Mining is universally profitable. Three separate production professions rely on mining these days: Blacksmithing, Jewelcrafting, Engineering. If you're going to mine, make sure to sell what you gather in ore form. It will sell more easily that way, as some professions need it in ore form (and those who need bars likely can get them made into it). Herbalism is a good choice too. Not as many professions use herbs, but because they're used in consumables, people will continue to need them.
Gather what you can, and sell everything at the Auction House.
Quest. You earn money (in gold, or drops) from the mobs you kill, and earn money as a reward for the quest. It's double-dipping monetary goodness.
Whether you're 70 or not, you can earn money just by using your resources, and focusing on supplying the market with goods instead of purchasing goods. Quest when and where you can, make frequent posts at the Auction House, and recognize a good deal when you see one. If you're unsure of an item's worth (recipe, rare item, dropped reagent), check thottbot or wowhead for "Average Price" and post the item somewhere in that range.
Use your head. Don't waste your money, real or virtual, on strategy guides.
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