Quote from Husky »Make sure you have a premium account, make sure you are not using a torrented download of minecraft! If you are you cannot go online.
Quote from Fooby »you buy the game... http://minecraft.net/haspaid.jsp?user=airdroid which you did.
You need to put in a SERVER address when you hit multiplayer. This isn't an MMO, people have to HOST servers for you to join.