Repair leaking geyser overflow

Please, define "geyser"...

Here we call a "geyser' a water heater. Several things would cause your problem.
1) Excessively high temperature or high water pressure. (have you got either one?)
2) If a check valve, backflow preventer or sprinkler were added. In which case a expansion tank is called for.
3)Trash lodged in the seat of the T&P valve, (see image) You might try to jiggle the handle and see if that helps. **OR**
4) A faulty valve. In that case simply replace the valve.
Good luck, Tom