Don't worry about being nervous, I still get nerves before heading out for a practice sortie, usually they are gone by the time I reach the practice area or after I roll inverted for the FOD and inverted systems check.
Acro IS unnatural and the forces effect people differently, the most important thing for you as a passenger and aspiring acro pilot is to commit to letting your instructor know as soon as you start to feel queasy, a good instructor will end the flight there since most folks don't recover right away. Sometimes having you fly (straight and level or easy turns, etc.) will help since it occupies more of your attention.
This is important to prevent you from possibly getting sick, which might keep you from trying again. Don't be afraid to knock it off early, and try another flight after a day or two. You really do build up a tolerance, and it can build quickly.
There is nothing wrong with admitting your gyro has spun and you don't feel like continuing, this is in fact a mark of good aeronautical decision making.
Don't go on an empty stomach, best to eat something light before the flight. I would suggest avoiding carbonated beverages but be well hydrated.
Ventilation helps alot, so make sure you know where the vents are and how to open them up.
Lastly, don't 'worry' about any of the above since that can be enough to become a self-fulfilling prophecy - just cover it in the brief that if you tell the instructor you are done, that is it, end of flight.
Take a camera, GoPro's are awesome, and really just plan to enjoy yourself. If you can get a list of maneuvers the instructor will fly let us know and maybe we can make suggestions - best is to have a plan since that helps you know what to expect. Hopefully the instructor will also help you with where to look since that can impact how your body interprets what is going on.
Have a great time! Acro flight is addictive, so be advised it can get stupid expensive.
'Gimp
