Stalls are the best (and most expensive) seats, followed by the next floor up (can't remember what it is called). It all depends on how much money you are willing to spend. The best seats will be the most expensive ones, and the worst seats will be the cheapest and furthest away from the stage. Price tends to be the best guide to how good a seat is. Be careful though - if you see some cheaper seats in a generally expensive area, it is probably because there is restricted vision from that seat (e.g. there is a pillar in the way). I have been to the opera a number of times and never seen any binoculars, or heard people shouting 'bravo' for that matter (actually I might have once...). People clap after songs and, sometimes, a particularly impressive vocal feat (if there is time to do so)... don't clap when someone is singing. ;) Oh, and read the synopsis if you want to have any idea of what is going on - the story lines can be very hard to follow if you don't know what is going on. During the opera the music is more important than the story line (it takes a long time to cover a small amount of plot). Enjoy the music, not just the singing but the orchestra as well.
People also don't dress that poshly anymore... but wearing jeans and trainers would probably not be the best look. I tend to wear smart trousers, a nice pair of shoes and a blouse (or something along those lines).
Opera isn't that different from seeing a play at the theatre as far as etiquette goes.
Have fun!
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We beat on, boats against the current, borne back ceaselessly into the past.