The first thing you need to do is stop the flow of water into the toilet. Remove the lid and pull the float up to stop the flow immediately. Then, shut off the water using the valve at the wall near the base of the toilet. This will give you a few minutes to breathe.
If the toilet is very very close to overflowing, you’ll need to find a bucket, cup or other device to remove some of the water. If you were able to shut the water off fairly quickly, fill the bucket or cup with very hot (but not boiling) water from the sink or tub. Pour it in the toilet a few cups at a time and wait a few minutes. The hot water should help break up the clog.
If the hot water doesn’t do the trick, try adding a surfactant. Dish soap, shampoo or even basic bar soap can help. Remove as much water as you can from the toilet bowl, then squirt a generous amount of soap in the toilet (break the bar soap up into small pieces), and repeat pouring hot water in. The soap should help lubricate the clog and pipes so things get moving.
If you still don’t have success with these methods, you may need to try to physically move the clog using a wire hanger. You can also attempt to “plunge” the toilet with the toilet brush. If the clog is close to being cleared, a few forceful pumps can generate enough pressure to move it through the pipes.
Once the water you’re pouring into the toilet is draining freely, turn the water back on at the valve and flush the toilet again. If you used soaps, flush several times to remove residue. The final step is to purchase plungers for everyone on your holiday shopping list.