We all know how to use a plunger. But if your toilet is blocked, wait until the water level has gone down as much as possible. Then use the plunger to try and push out the blockage. Flush with a bucket of water so that you can control how much water goes into the bowl.
You may need to repeat this a few times. With some patience, you should be able to remove most blockages.
NOTE: To prevent toilets from clogging, only flush items that are actually flushable, such as human waste and toilet paper. You should also flush only small amounts at a time, so keep the kids from trying to flush huge wads of toilet paper.