The repertoire of opera and vocal music is quite wide so just because of pure unknowing it's difficult to recommend. I quess Scarlatti's "Gia il sole dal gange" is quite popular among beginners But beginners usually start with vocalises(short singing exercises/pieces accompanied with piano, which include basic singing elements). And simply if you have lower voice, then you might be alto, if high voice then soprano or if you can handle both registers then you might be mezzosoprano. It's best if you'd see an expert, who can tell you, who you really are.
which include basic singing elements- basic for opera singing, so it means, these are not easy;)