start off with either a ninja 250 or ninja 500, they are the best to start on, easy to handle and are great on gas. They are also very cheap to insure, and retain their value, but DO NOT go higher than a 500cc for your first motorcycle. Just ride a 250 or a 500 for a season and when your comfortable you can trade up for something more powerful.
Well i got me a 900 for my starter bike, (99 kawasaki zx-9r) and i LOVE it, it has so much power to crave and lust after that u wouldnt want anything smaller than a 750 , 600 is fine but trust me youre gonna want more, be cautious of the power and drive responsibly!