Tips on singing? Help!?

My sister and i are going to sing at my brothers wedding, and i kinda want some tips on how to you know sound as good as possible... we're singing " A page is Turned" by Bebo Norman i think... i am a soprano and sometimes i can't really reach the high notes, but sometimes i can.. why? what can i do to make sure i can reach those notes in the song when the time comes? what are some tips on singing etc? Thanks!