How do I fix a toilet tank that overflows when you flush the toilet

Are you quite sure that it isn't the bowl that overflows? While it's possible for a toilet tank to overflow it's very unlikely. Septic tank or city sewer? Was anything dropped in the toilet?
Now if it's the bowl that overflows you will need to get a closet auger,(see image) and auger out the bowl. Good luck, Tom

It is the tank that overflows - through the flush lever area. Water rushes into the overflow tube and continues past the point of filling and then leaks through the flush lever area to the floor.

If it is the tank that overflows it sounds that your water level is high and you can adjust your water level, please see the images below and tell us which fill valve do you see inside your water tank and we will tell you how to fix yourproblem, Thanks.

John