If you are interested in buying Paris opera tickets for 25 euros each, or ballet tickets which may be less, you can look at the calendar at www.operadeparis.fr for the time you will be in town.
They have an English web site but not everything is fully translated; I suggest using Google Chrome's translate feature as needed, along with a separate window opened to google translate so you can copy and paste text as needed. Also it is a good idea to navigate around the site so you know what you will be buying and have a good idea of the seats, etc. I suggest being online and preferably pre-registered with an account before the tickets are released.
Be sure to check the web site as soon as you plan your trip, and then mark on your outlook calendar the day they will go on sale: They release the tickets on one day at 9:00 a.m. Paris time, which is midnight California time, for a block of dates lasting a few months. In our case the showing we saw was sold out by the day after they went on sale (although it was not full when we were there, so there may be last-minute tickets available somehow, but I can't speak to that.). At 9:00 a.m. when you click to buy tickets, you will be in a queue and your screen will be mostly white with a timer in the middle (in French). You just need to wait until your time comes up; it took me 15 minutes. Don't navigate away or you will lose your place in the queue. Then when your time comes up you can go ahead and book your tickets by date, time, and take the first seats they give you in the price range you request, or else the next seats will be worse (as was my experience).
Also, note that the classic opera house is the Palais Garnier. the Opera Bastille is a modern opera house built in the last 20 years, so it may not be the experience you had in mind.