What would be the kind of sports bike apt for a beginner to start-off?

I'm new to riding bikes, and I've ridden cruise bikes(
I'd like to start-off my sports-bike riding. For now, my options for a beginner's sports bike are a Kawasaki Ninja 300, a Honda CBR250RR and a 600RR.

I could afford a budget around 6,000 USD to 7,000 USD.

What, according to your opinion and experience, would be a good starter sports bike for a beginner? And what kind of technical know-how and riding skills should I be needing to work on, in order to use one properly?

Of the bikes listed and your stated experience I would recommend the Kawasaki Ninja 300. The Suzuki GW250 is 24 hp. The Yamaha 250 v-star is 21.5 hp. The Honda CB250 is 25.5 hp. The Ninja 250 is the most powerful 250 rated at 26hp. The Ninja 300 jumps to 36hp. Quite a design feat for a 50cc gain in displacement. You could also consider the Yamaha r3. Rated at 42hp. Actual displacement is 321cc. So, 21cc more than the Ninja 300. But, these two bikes are not true sportbikes like the 600rr. These two beginer class sportbikes are designed for the beginner with less experience than you. Things like the front forks are slightly out, slightly more upright riding position, and a less agresive power curve on the engine. And, yes less overall power to keep you from over excelleration error. The front forks being out slightly makes the bike much less prone to high speed wobble. The slightly more upright riding position makes you more visible to other users of the road and makes you more alert to your surroundings. The less aggressive power curve stops a multitude of throttle input errors common on a true sportbike. Not to mention that the less agresive engine also starts easier in cold weather. Just an added bonus of reliability. My favorite would be the r3 at 42 hp. Mainly because it has a more rounded power curve. Designed with a cross plain in-line two cylinder. Giving you more low rpm power and smoother running at top rpm. Slightly less overall power than the typical in-line two. So, Yamaha sets things into balance by adding 21cc. For a total of 7 more hp than the ninja 300. Yes, Yamaha cheated by making it larger than the 300cc ninja. But, the fact that it weighs less than the Ninja should be an indication of overall design. As far as your experince goes and wether you should go for the 600rr. If you doubled the hp for the ninja 300 the 600rr would be 70hp. Since, after all it is exactly twice the displacement. But, instead the 600rr is 110hp. Honda and other manufacturers can acomplish this by putting all the power at a very high rpm. But, this sacrifice gives you little to no power at idle and low to mid rpm. Mandating that you keep the bike reved up for all rididng. Even leisure cruising. Which is kinda frustrating if you just want to sit back and take it easy from time to time. But, there's an alternative. The Yamaha fz7. Built with the same cross plain engine design as the r3. But, with more emphasis on low to mid range power. In my opinion a better choice than the 600rr. Unless, of course, you want to race around on a trac.

Not going to recommend a 600 RR but will mention that there are bikes in the lower-mid- range like the Honda CBR 500, good powerwithout being overwhelming.

How about the cb300f?