Find out what you're seeing. In musical terms, the phrase "Hey, guess what! I've just been invited to the opera!" conveys about as much information as "Golly! My friend has just asked me to attend a sporting event!" Specifically, ask the person who invited you what opera you'll be seeing, who wrote it, whether it has any famous melodies or scenes, etc., anything that can give you a firm, non-abstract grip beforehand.
Do your homework. Okay, let's say you've been invited to see Puccini's "Madama Butterfly." If you go on the web and do some research, you'll find out that a) "Madama Butterfly" was written at the start of the 20th century; b) it's set in Japan, and features an American character, a rarity for the opera of its day; and c) it's the source of at least one tune you're likely to be familiar with, "Un Bel Di" ("One Fine Day.") You'll also discover that "Madama Butterfly" is an accessible, melodic, and hugely popular opera that has been in the worldwide repertory for over a century.
Sample some music. The reason aficionados attend the same operas over and over is that they're intimately familiar with the music, and thus are in a position to judge how well the orchestra is playing and how well the singers are singing. It's unlikely you'll want to buy a CD of "Madama Butterfly" beforehand, but you should be able to sample some streaming songs on the Web, which will at least let you know what you're in for.
Dress well-and be patient. Very few operas feature car chases and explosions, so if you're the type who can only sit still through a Jerry Bruckheimer spectactular you may want to reconsider your plans. However, if you have even the attention span of a 12-year-old, you should have no trouble sitting quietly, watching, listening to, and reading the English supertitles of "Madama Butterfly." Remember, the plots of most operas develop very, very slowly-that's what gives them the emotional depth that Andrew Lloyd Webber musicals so sorely lack.
Keep an open mind. If your first visit to the opera has left you bored silly, bear in mind that operas come in all shapes and sizes-and a fan of Mozart's "The Magic Flute" will be unable to tolerate an atonal opus like Berg's "Wozzeck." It's possible that you may have seen the wrong opera on the wrong day, so don't write off the entire form just because you've had an unpleasant experience. (Hopefully, of course, you'll have had a great experience, in which case a whole new world of bosomy divas awaits your patronage.)