I want to play Guitar Hero 5 but it says I need a microphone?

The reason why it says you need to connect a microphone is because you use your PS3/xbox 360 controller to get to career mode. When you use the ps3/xbox 360 controller, the game sets itself that you are using a microphone because microphones, unlike drums and guitars, don't have built in controls. Use the guitar to get to the menu (the green button is the X/A, the red button is the O/B). This ought to solve your problem.