Does anyone here have iRig (microphone for iPhone)?

If so, i need some help! I've bought it because I looooove to sing and record and thought this would be cool. But I'm not much of a computer genius so i was wondering: what apps do i need (free or not free) to get the iphone to "erase" the vocals and have me sing over the base of the songs? And how do i set it? Thank ya!