Since you never hunted before,
I recomend you hunt rabits on your own without the dog first.
Get yourself comfortable with the firearm, woods, animals your hunting first.
This way you can teach the dog, and not follow it around having it teach you.
Also having a dog out there is another factor increasing the odds of a mishap, don't want to injure your dog as your learning.
With a dog your shooting rabbits on the run.
Using a rifle will make it very difficult to shoot on the run. Will want a shotgun for shooting with a dog.
Others can say which is best, .20 ga is probably biggest you should go.