Go ahead and sing any Love Live song, off vocals are on the wiki.

http://love-live.wikia.com/wiki/Main_Page

I suggest singing Aishiteru Banzai, Start Dash, Korekara No Someday, Susume Tomorrow or Bokura wa Ima no Naka De, but float your boat with what you like best. You don't have to sing the full song, just a portion for me to get a general understanding of your voice. No need to put off vocal if you don't want to.