Been there. for various reasons I was no stranger to vomiting during my flight training.
Luckily never actually in the plane.
I made the epic mistake of thinking that if I didn't eat . I couldn't be sick
WRONG! It makes you feel a million times worse. Find a light meal that works for you. For me it was granola and greek yoghurt. carbs and protein. Oh and keep hydrated as well.
Make sure your instructor doesn't keep the heat up too much either. My instructor and I existed in different temperature zones. I needed it cool and he liked the heat.
we had to find a compromise but cooler is better when you are feeling a bit bleurgh