How do you speak to a NPC on WoW?

If an NPC doesn't have anything to say or offer, that's all you will get. Now if there's a quest or something there will be further options for you to communicate with the NPC. You can't just walk up to any NPC and start conversation at any given moment. Also city guards will help you find most of the things you are looking for. You can visit the wow forums @ battle.net/wow for help on various topics and issues.