Breathe! Everything will be fine, especially if you've been doing your research. Just take a moment to clear your head and the knowledge will come back to you. Monitor the mommy and baby's vitals, make sure the food has been properly adjusted to care for the baby (which is something that matters a little more earlier in the pregnancy) and remember that animals do come equipped with the instincts to care for their young. If this is her first baby, you will need to watch more carefully because the chances of a rejected joey are higher, but keep track of everything and I'm sure you'll be fine!
I wish I could give you more info, but I don't have much experience with joeys myself. Good luck, though, and congrats! Babies are so exciting!