Congrats on your first job! There are a lot of great posts on
this thread- check it out, if you haven't already.
Check in with your SM about any cuts to the score- they will affect your timings. Usually these are decided before rehearsals begin.
It sounds like you have familiarized yourself with the score by doing your tabs, which is great. If you can tell from the score when certain singers enter, I'd pencil it in, even if it's just an approximation that is adjusted in rehearsal. It's helpful to know this so that you can make sure they are ready when you are approaching their scene. Anything you can do to make yourself familiar with the score will help you- I will listen to it in the car on the way to rehearsal.
Will you be in during "prep week"? That's a great time to familiarize yourself with the costumes, and figure out which rehearsal items are needed. Shoes, skirts, and jackets are always helpful, as well as any items like hats or gloves that may be taken on or off. Be prepared to hand off these items in rehearsal, including things like rehearsal skirts- they are more likely to use them if you have them ready.
Get to know the costume shop staff- they will be an invaluable resource for you.
One thing I have found helpful is to figure out how far into the opera each character's first entrance is. If Characters A-L are onstage at the top of show, and Characters M-P don't enter for 45min, the dressers can prioritize the earlier entrances when helping folks get ready.
If you're an ASM that will be cueing entrances, once entrances are set, put a 5 minute and 2 minute "warning" for each entrance in your book. If the singer is not there when they should be, this will give you a few minutes to figure out where they are, have the SM page them to backstage, or (if all else fails) run back to the dressing room to get them.
Don't freak out- I'm sure you'll do great.
Ask questions- your PSM can give you the best information about what they expect from you and what you can do to prepare. Best of luck!