Hey, I'm a voice major. Word.
So, the most important thing is to listen to someone who is a native italian speaker sing your piece. That being said, I don't necessarily recommend singing Se Tu M'ami because a lot of people will do it. However that being said if you stick with it, like, bwaylv song said, it's really flirtacious. The girl singing it is basically saying "I love you, but I love him and him and him and him...so...."
If you decide to go in another direction, some other good ones are O Del Mio Dolce Ardor or Tu Lo Sai.
You also might have a better time singing in french if that's what you take in school. If you take spanish stick with italian, they're really similar.
Good luck, you'll do great!
Cecilia Bartoli