Tips on singing lower notes, please?

I am auditioning for my school's talent show in a few days with Josh Groban's "You Raise Me Up". I have a good voice but I have trouble reaching the low notes in the first line, "When I am down/And oh my soul, so weary". Whenever I try to sing the line, my voice croaks and is barely distinguishable. Can someone give me tips for a sure-fire way to achieve those notes? I really want to leave a good first impression on the judges.