Not being a pro myself, and not knowing what you are shooting I am not sure who will be able to say what to expect... Wedding? Portraits? Event? etc...
As to what to take; as many spares as you can gather your hands on, everything checked, cleaned, tested, checked again. Put a list in your bag of everything you want to take with you, checkboxed to show you have checked it and are happy with it. How about a little list of what to check on the camera just before shooting, i.e. shooting
mode, aperture, shutterspeed, white balance, focus
mode, expsoure compensation settings , ISO etc just to make sure you don't make that blunder that some of us do from time to time in the excitement...
Above all the obvious camera related gear, make sure you are dressed appropriately for the shoot. Sometimes the impression you make is far more important than anything (you will get the shots you want I have no doubt).
And the getting paid bit... I would accept anything the pro wants to pay, this will get you used to taking responsibility for your actions. Realising that you are being paid for your work, learning or not, helps put a level of perspective and pressure on you that you will have when you start out on your own.
Brett