How do I turn off the water flow to my toilet?
In almost all cases there should be a shutoff valve behind the toilet, just follow the visible pipe that comes out the bottom of the toilet tank. It will either be a globe valve that has a handle you need to turn many times to open/close, or a ball valve with a handle that you turn 1/4 turn to turn on/off.

If there is no shutoff valve, you would need to turn off the supply to your entire house.  It would also be a good time to install a dedicated shutoff for the toilet.