then I tend to agree with SimplicityInTheLBC. I would suggest you stay away from Wagner as your first opera. His music is heavier, less approacable.
And while most operas are tragedies, Puccini’s (Madame Butterfly, La Boheme, Turandot, etc.) music is incredibly gorgeous. La Boheme has some of the most beautiful arias ever written, though it does indeed end in tragedy.

Mozart is wonderful, some of the best vocal ensemble music ever compsed, but his operas tend to be long, though not as long as Wagner’s are. Plus, Mozart’s operas, written in the 18th century, are of a slightly different style than the famous operas of the 19th century. Some of his operas have spoken dialog in them whereas operas of the 19th century composers (Verdi, Puccini, Donizetti, etc) you would never see that. (Actually Puccini wrote some of his operas in the early 20th century ).

I personally don’t go to the opera for the stories, I go for the music.