Having experienced it myself, I think I can offer a few words.
When I was a student my instructor started me off with relatively short flights, about .6 on the hobbs. We gradually worked up to an hour over the first three or four flights.
For me, a large part of my nausea came from vibration. We had two 152s where I learned to fly, and after flying both I found that one vibrated more than the other, so I tried to always fly in the better airplane. Also, slow flight tended to make me the most sick, and I think the increased vibration during the maneuver (slow airspeed with high RPM) was the cause.
For me, sometimes I think I got more sick when it was smooth, because the tactile inputs were missing. What I saw and what I felt did not agree. When it was bumpy (but not very bumpy), what I saw and felt usually did agree!
I never did have to use a sick-sac in a plane. The only time I felt I really needed it when I flew in the back seat of a plane with a friend of mine and his instructor. If you feel that you are going to need one, you need to get down. I would not recommend waiting until you get sick to determine it is time to land.
After my first few lessons where the nausea started to pass, but for about my first 10 hours, driving in the car on the way home after a lesson usually made me more sick. Wait about 20 minutes after you land if you find this is the case.
You can buy ginger capsules or candied ginger, it is supposed to help and I used it for my first 20 hours or so. If you decide to eat it in the car, make sure you have some water...it burns!