Its really easy and should be done right from the start of the game. Grab the 2 primary professions, mining and skinning. While you're out leveling yourself you can easily skin the animals. You're bound to eventually enter a mine which will have many minerals. Be sure to mine those ores. You can easily sell off the extra items to merchants or players.While you're out leveling, there are certain monsters that has better drops than others. An example would be humanoids. They tend to drop more gold and items than any other creatures in the World of Azeroth.Don't spend any money buying World of Warcraft items, equipments and other accessories early in the game. Low level characters from 1-40 are not gear dependent. Along with that fact, you'll get a nice load of items from just completing quests. 

Make sure you grab quests every chance you get. You can easily gain additional exp, gold, items and faction while you're leveling. You may even complete some of your quests with out even knowing because they normally require you to kill off mobs or require you to travel/speak to other NPCs. The quests of World of Warcraft are more player friendly than other MMORPGs.