I have been to live opera a few times, and have seen a number of them on TV (PBS). I used to hear opera as a kid, because my mother always listened to the Metropolitan Opera's Saturday afternoon broadcasts. (Those are still on, by the way - check their web site). As a child I never knew what the people were singing about, and my mama didn't tell me - there's a reason daytime TV dramas are called soap OPERAS - but I came to enjoy the music.
If I were you, I'd check with the performance venue and ask which opera they're presenting this season they would recommend for "beginners." Then look that opera up online, bone up on the plot, and see if you can listen online to some of the numbers. That will prepare you. Oh, and ask if the venue does English subtitles, because most operas are not sung in English.
Go for the adventure, as you would when you try anything new (and you seem to be an expert at that - good for you!). Wear comfortable nice clothes - it isn't necessary to dress up like Hollywood. If your seat is far back, bring some binoculars. Don't start passing judgment on the production too quickly. Keep looking for anything you like - the melodies, the costumes, the characters, whatever. Use your good theater manners, but if you happen to be sitting next to knowledgable people, pick their brains during the intermission if you can; ask how they got interested in opera, what they like best, etc., etc.
Have fun!