If you want to get into opera, get into the most popular ones first, and take it from there.  Giuseppe Verdi is a very prolific composer and this is an excellent version of La Traviata (The Lost One).  My dad always said Joan Sutherland was great, but I had no idea just how.  The DECCA recording of La Traviata with Sutherland, Pavarotti, Manuguerra and the National Philarmonic Orchestra with Richard Bonynge is just superb. Get the full version, on 2 CDs.  It's sublime from the prelude to the end.  Conveniently also on iTunes.

Buy other versions, or just the arias, and compare different tenors.  I still think Pavarotti is unmatched.