Before you do anything else, resist every instinct to flush a second time. The problem isn’t that you didn’t flush hard enough; it’s that the exit is blocked. A second flush will only add more water to a bowl that is already unable to drain, guaranteeing an overflow. If the water level is dangerously high (touching the rim), you need to act fast: remove the tank lid and push down the flapper (the rubber valve at the bottom of the tank) to stop any more water from entering the bowl. Then, be prepared to bail water out with a small bucket or a cup into a larger bucket or a bathtub drain.
Before attempting to dislodge any blockage, you must control the water flow to prevent a hazardous bathroom flood. Unclog Your Toilet Without a Plunger - Super Home Warranty fix clogged toilet