If you are really clueless (I mean that in a supportive way) then turn the water off. If you know what you are doing just turn off the supply to the toilet. (in our house there is a pipe very near to the loo, and half way up it there is a kind of screw which I give a gentle half turn with a screwdriver.) If you can do that then you will still be able to use all of the other taps etc in the house.
If that sounds too complicated, just turn off the entire water supply (usually a tap under the kitchen sink, well everywhere I have lived it has been there) hth.
If you go for a wee in the night, don't worry. You will be able to either flush once (but it won't fill up again obviously) or just leave it.