What are some ways to get my voice to Sing Higher?

So I wanna write lyrics to Zelda's Lullaby and sing it for the talent show at my skool either this year or next (it's in April and it depends how long it will take to get the notes right). I'm in Soprano in choir and probably have the highest voice in my school. But I just can't get those really really high notes and no one really can. What are some tricks to get my voice healthier and be able to sing the Zelda High notes?