Congratulations! It’s a wonderful job - I’ve been one for over 30 years, and I love it.
I’d say the most important thing is to be comfortable with the Psalm verses, because here’s where you’ll be singing alone. If you can go over them two or three times (on different days) during the week ahead, you’ll be just fine. The more different times you sing through them, the better, because your mind continues to work on it while you do other things.
Spend a few minutes when the church is empty (so you don’t disturb anyone) checking how to use the mike, and have the organist tell you when the sound is good for both speaking and singing.
If you’re being paid, be sure you know where to sign in, or if the music director keeps track for you, or whatever.
Check with the celebrant before your Mass, so you know if there’s anything different or special going on. If you have to do announcements or anything, read through them beforehand so you don’t get surprised with a weird last name or some other issue of pronunciation.
Have fun! It’s wonderful you’ll be able to do this.