There is no such thing as a Beginner bike, it fits or it doesn't. If it 's too small it's easier to crash than a bigger bike, but bigger bikes , especially sport bikes like Ninjas take a quantum leap in power.
I've ridden both cruisers and sports, cruisers are better for beginners, they are more rider and Insurance friendly.
###1 take a safe rider course. they teach everything needed to ride and pass the exam.
BUY a used , GOOD bike, if it's been dropped a few times that's OK, it shows Character.