How do you get to level 507 on World of Warcraft?

What do you mean? You level up by getting experience from killing creatures and doing quests, which you level up with. You just get enough experience to get past level seven!

If that's not what you mean, then i don't know. Your character's levels are the only 'Levels' in the game, this is not a game like Mario or something where you have different 'levels' to complete!