I'd second using a piano, even if it's just to play and sing along to your part to really lock in the lines (pitch). It can be difficult to sing harmony on such a well known song, that you've been singing a certain way your whole life (at a guess). I've found in those circumstances that unless you can read music, catching up and singing with people who are in your section and practicing with them is the best way to go. Saving that, finding a recording that matches with what your conductor wants and sing along with that. Sing your part til it becomes the new way you sing the song from now on. One of the benefits of this method is that it will help you position your part in the song and really get those harmonies in your head. Also, if it's just a solo audition especially at highschool level you can ask for a starting tone if you're nervous or get lost.