Is it "Leavin'"? If so, that would be Jesse McCartney.
Don't stress, don't stress, don't stress.
I'm not a big fan of today's tween music, but that song started to grow on me not too long ago. And that Jesse McCartney sure did grow up nicely! Apparently he's 21 now.
Whenever I'm having a hard time finding a song and I don't know the lyrics, I usually visit the iTunes store and search through 'Top Songs'. If it's something that's currently playing on the radio, chances are it'll be there.