Do not be tempted to go on an empty stomach, eat normally. Be hydrated, and call 'knock off' the moment you feel queasy. Sometimes taking the controls yourself and staying level for a couple minutes will allow you to recover for a bit, but once the process starts it does not go away, especially for first experience.
If your instructor is a good acro stick he will know all this and will keep it smooth and positive except for maybe a 10-15 second level inverted. Loops, rolls and barrel rolls, even spins are all pretty mundane and good choices for an intro flight. Hammerheads, tailslides, lomcevaks while very fun once you have a tolerance can be very disorienting during a first flight.
The key to enjoying it is calling it off once you feel remotely uncomfortable.
When you get down, unless you are seconds from puking, hydrate again - preferably not carbonated (e.g., I keep water and Gatorade in the hangar fridge for post Yak hydration). Walk around and work out the adrenaline, again unless you are ready to blow chunks in which case find a place and let nature take its' course. No shame in losing your lunch - we are not naturally inclined to handle the gyroscopic and g-forces experienced in acro flight.
Most of all, don't focus on getting or not getting sick, focus on having a good time. Ask questions, talk during the flight - ask your instructor to tell you where to look since that helps, alot. Before you take off, have him demonstrate his preference for anti-G straining maneuver, and practice it. Don't be surprised if you find your legs shaking while straining, mine still do.
Have a great time!
'Gimp
