The most effective ways to get Gold are hunting, questing and crafting. When you hunt, the enemies you kill drop items, and even the most useless ones can be sold to vendors for money. Quests on the other hand give up rewards in money and items, the money part is most useful as it is usually a large sum. Crafting is also another alternative for earning Gold, you just choose any 2 professions and use it to gather raw materials or create items which you can sell to vendors or players. Items sell higher to players since vendors have a set price and people always want to buy at a lower price than the vendor but sell at a higher price, so there usually is a middle price. To see what the going rate is, type in "PC" (Price Check) in the Trade Chat window and the item you want to price check and someone should reply with the going-rate for that item.
Many items are rare in World of Warcraft and a lot that you pick up is usually soulbound to you, meaning the only way you can make any money off of it is to sell it, here we list the 20 top selling items to NPC vendors: