How do you get to another level in Warcraft?

Answer 1

You fill up your xp bar at the bottom of the screen, above the interface, the bar with the parallel lines on it. You fill it up by killing various monsters and doing quests, each which give a certain amount, which may vary. You cannot gain xp from monsters 5 lvls below you until lvl 30, and you will gain only a tiny amount or no xp from quests whose name is grey in your quest log. Also, the higher lvl you are, the more xp you need to lvl up.