Ah, welcome to the dental World.
Be welcoming and hospitable. Give clear instructions and directions for any tasks.
There will be problems with the diary changing and appointment availability; do your best to iron out the wrinkles, but some problems cannot be satisfactorily fixed, and that will be out of your hands. Some people will kick off at you. Don't internalise that, their emotions are their problem to deal with, not yours.
Most people are lovely but may be scared. If someone is especially frightened as they come in, see if you can give the clinical staff a heads-up (our software package has an instant messaging system to communicate with the surgeries). Sometimes the person on reception gets the most honest story!
If you're unsure of something, tell the patient you'll check with a colleague and come back to them. People are fine with that as long as you do come back; keep a to-do list beside you and write things down, because otherwise you will get home and remember what you forgot.
Ask questions as they arise and don't feel awkward about it. You're new, your colleagues will be expecting to teach you and support you.
Good luck for today, hope it all goes well!