Please help ! Gotta sing better within 17 days !?

well i gotta sing and play the guitar on the Bruno Mars song 'Count On Me' at school aftter 17 days. I am very good at playing the guitar and am well versed with the songs chords. But i cant sing that good coz my voice gets shaky at times and thats the biggest problem. Otherwise,i can sing the song well. How am i supposed to overcome this problem,and can you all please give me tips on how to sing well on a mike and any sort of vocal exercises ? Please help.