First, turn off the water supply to the toilet. On modern plumbing fixtures there is a shut-off valve just under the tank. For older fixtures you will have to shut off the main water supply to the home.
With the water off, flush the toilet to remove most of the water from the tank. Use a sponge to remove the water that remains. Next, disconnect the water supply line to the tank using pliers or a wrench. While you're down there on the floor, you will notice that the tank is fastened to the bowl with two brass bolts in the bottom of the tank. Remove the nuts and washers on the bolts on the underside of the tank and remove the tank.
Be sure you have a soft surface ready to sit the tank on. The tank is a ceramic material that breaks easily. Repair the wall and replace the tank.
You will probably need two new tank bolts and a new tank gasket. Kits for toilet repairs are available at most home stores. Turn the water on, check for leaks and then flush twice.