When we went we also got unassigned seats. We arrived fairly early to make sure we could get a spot of our choosing. That said, there really are no bad seats. It does fill up though so get there well before 9. If you sit off to the side, rather than facing the stage head-on, you'll be closer to the performance.
If you're in the upper area, which I think you probably are since you have unassigned seating, you'll be sitting on the stone of the ancient arena so bring something soft to sit on. You can buy refreshments from people that walk around the seating area, kind of like an American baseball game, so you'll be able to get a snack and something to drink while you wait.
You'll love seeing opera in this venue! It's a unique experience in the musical world!