Microphone/Voice Changer?

I'm trying to play a prank on a friend of mine. I'm going to make a fake Skype or ooVoo account and pretend to be a guy (I'm a girl) and I need to know how you can do this. I know I need to download a program and buy a microphone, but what else do I need to make this work? I don't want to screw up, lol.