Welcome Derek!
Being new is sooooo much fun because everything is a fresh experience.
Your tire question...
https://faq.ninja250.org/wiki/What_t...fit_the_250%3F
The BT45's or Sport Demons are standards on these bikes.
As far as other mods I would make at this time would be continued mods to the rider's skills and knowledge. Poke around this wiki
https://faq.ninja250.org/wiki/Ninja250_Howto and get familiar with how your bike works and learn to do the normal maintenance items (oil, brakes, coolant, ect...). Hmmmm... not sure I have ever seen any sliders for a pregen, but I haven't really looked either. If you must make a mod, flush mount front signals can save you some cracked fairings if your bike falls on it side.
Good luck! Have fun!