I was in your shoes not too long ago. To get started you’re going to want a basic socket set, a matching ratchet (i.e. the drive size of the ratchet should match that of the sockets), some equivalent combo wrenches, some allen keys, and a torque wrench. Everything should be metric since you have a Kawi (i.e. meters not feet/inches). You’ll need more tools as you try to do more complex things, but I would recommend buying them on an as-needed basis. I haven’t used 90% of the tools in my tool kit.
You’re also going to want a front/rear stand for your motorcycle so that you can get the bike stable, remove wheels, etc. I recommend Pit Bull. If you don’t want stands you could probably go with a motorcycle lift instead, but that certainly won’t be cheaper.
Hmmm…I know I forgetting stuff…
Oh, and when you’re measuring oil in the sight, the bike needs to be vertical. If I remember correctly you have an older Ninja 250. Those have center stands, right? Just get it on the center stand, make sure it’s off and cooled down (five minutes post-ride should do), and then take a look.