Duck, dont let it put you off at all, I'll quite happily lay it on the line and admit that I was by no means a natural when it came to keeping my lunch down. Many a time in the last 15 years have I either felt sick or thrown up, some of the more memorable examples:-
First flight in a chipmunk as an Air Cadet, I threw up.
Third or fourth flight starting my PPL, I threw up. (and felt sick most times before)
Instrument training over the Scottish mountains on a bumpy day under the hood, I threw up.
PPL revalidation flight on a hot day, dehydrated wearing a big jumper, I threw up.
CPL unusual attitude recovery on instruments in 40C heat in Florida, I threw up.
Jet Provost conversion Training afer an Aeros session (quite a bit of negative G), I threw up.
Getting carried away stringing too many aeros together in my Tipsy Nipper, I nearly threw up.
Think you'll be getting the picture by now....!
Now what I've realised after filling a number sick bags over the years is thats its usually a number of factors and not just flying itself. In my early days of flying it was typically exctment / nervousness and paranoia about being sick that made me sick. Once you get further into your flight training your confidence will grow and you'll be far too busy to even think about feeling sick!
IMC training on a bumpy day at failry low level I put down to poor planning by my instructor at the time.
Overheating on a hot day, as suggested before, wear light clothes so you keep cool, keep some fresh air coming into the cabin and keep yourself well hydrated.
As for aeros, that just comes with exposure, by the end of last summer I got to the point I could happily fly a full aeros session without feeling sick at all. After having a break from it for a couple of months and then going back I felt a bit iffy after only a spin, a couple of loops and some rolls.
So basically dont worry about it, your body is beinig put in a totally unnatural envoronment and will take some time to adjust, rest assured though it will adjust.
You can help by getting some Ginger chewing gum (you'll find it on the net) and I also tried a wee wrist band thing that gives you a small electric shock every few secs which helped during the early stages of learning Aeros. Try shorter flights initially so the sickness doesnt have time to get so bad, this will also help build up your confidence and stop you worring about getting sick etc (which will make you feel worse!)
I'm sure lots of others will be along with good advice but the main thing is not to leat it get you down and to know that it will get better the more you fly!
Good luck!