i don't know how to just sing on ps3 rock band?

What do you mean you don't know how. The lyrics scroll from the right to left and the little lines above it are for pitch. The higher the line, the higher your voice goes. Instead of them going verticaly, like all the other insturments, it goes sideways, on top of your screen if you are doing band mode. Then you just sing the words when they reach the "line," that's the only word I know to describe it, I hope you know what I am talking about.