If the throttle is frozen, taking it off the handle bars is pretty simple. If it has sat covered for a while, something might have made it’s home in the space between the throttle and the handle bars and crudded it up. While the throttle is off pull the push and pull cables and observe how they act down at the carb. Squirting a little WD-40 down the sleeve at both ends and working the line would help alleviate problems in the sleeve, if there are any. My bet is the throttle is seized and a disassembly and cleaning would help out your situation a good bit.
If it’s anything like mine (Kawasaki KLX250), the hardest part is keeping the pull cable in place when putting it back together. Just remain patient and it will go smoothly. I know the ninja 250 has the same carb setup as mine.
Good luck, and if you don’t feel confident doing it yourself take it to a shop.