If you don't want to buy World of Warcraft gold online you can get it the way the game intends you to. You can get gold by hunting, questing and crafting items for sale to other players. When you go hunting most creatures will drop some currency and items as loot. Most if the loot is junk and can just be sold to vendors. Quests though are a good way to get gold. The reason is that you can get some high level items that have a high value. some of them you will be able to use on your character. Other items that you can't use that are green, blue or purple should be put up for auction to be sold to other players. The last way you can make gold in the game is to craft items for resell to other players. The higher your crafting skills are the more you can get for the items created. A good crafter can profit 500 or more gold a day.
Another comment on questing and getting good items as loot. Many of the items are soul bound. This can be a pain because much of the stuff you can't use. But the really good items can still be sold to the vendors and you can make some good money at it. Here's a list of some of the more common soul bound items you can find and what the vendors will pay for them.