Stay away from liquids, especially coffee. Fill your stomach with plenty of liquid absorbing food, bread, crackers, mashed potatoes, whatever, just don't go on an empty stomach. If you find yourself getting a little anxious, don't worry, that will pass for good after just a few flights.
Your going to love flying once you accept it is more about following the rules to the letter, than actually aviating. Cross your tees, dot your I's, take nothing for granted. Do exactly as your instructor tells you. Flying is about the most regimented, rule governed, endeavor you will ever involve yourself with, unless of course you are an engineer or going to work for the government.
Good luck.
John