Best way to start really is with something shorter with universal themes that are very easy to understand just by looking at what's happening on the stage or inspired by traditional stories you already know. I'd recommend Mozart's Don Giovanni or Puccini's Madame Butterfly as both are easy to follow and equally appealing to either a novice or a purist that's been to hundreds. Generally anything by Verdi, Mozart or Puccini are good as their works are widely appealing and regularly performed. It's always worth starting with something simple though if anything just because it gives you an idea what to expect and if you like it, more of an appetite to watch heavier, more conceptual, more abstract, longer operas that are exhausting even to the already initiated. Generally most of the national/touring ones like the English National Opera, Welsh National Opera and Opera North are great to start with because they're more affordable, often (nearly always with the ENO) performed in English or at least with English surtitles and try to appeal to newcomers. If you can too it's worth trying to get an idea of what kind of staging it is, more traditional ones are much easier to understand than some modernist/abstract stagings that take a lot of artistic licence and don't always make sense or resemble the originally intended setting or plot. The first time I saw Wagner's Tannhauser the music and performance was great but I genuinely didn't have f***ing clue what the plot was meant to be until I saw a traditionalist staging rather than the post-modernist version I saw first at the Royal Opera. Despite the words and music you still have no idea what you're watching is supposed to be a myth set in medieval Germany when most of characters look like they walked straight out of a Balkans conflict era militia carrying mock AK-47s in ruined theatre that gets more destroyed as each act goes on.