Hello and welcome!
The standard answer is going to be a small CC Kawi Ninja, e.g. EX500. They're cheap and great to trash on since they're not too expensive. Since it sounds like you'll be using it as a stunt bike you prob. want to be using as cheap of a bike as possible, esp. to get your feet wet on two-wheels. Might want to search the Stunt sub-forum for info on engine cages and such.