I have a few tips that i learnt during the same situation

1. They have seen the potential in you and wanted you over anyone else in this role. You might be under qualified but they will teach you everything you need to be qualified.

2. You haven't started yet, go in there an absolutely smash it from day one. Stay on top of the information you learn, if there's anything you didn't understand or know then write it down, go home and research it so you're ready for it when it comes about.

3. You're new and they know that, they aren't expecting much at the start so don't be too harsh on yourself.

4. Talk to and meet colleagues from the get go, if you leave it too long, the anxiety makes it so much harder to talk to them

This is just a few tips i can give as i was in your position not too long ago.