A small leak in a galvanized pipe can be easily fixed with a wraparound repair couplet. Anything larger will possibly require replacement of the pipe and the use of new fittings.
There is no temporary fix for any leaky pipe that amounts in any satisfaction. The pipe should be replaced by a professional. You can temporarily seal the pipe with a band clamp and a rubber patch, but this will not hold forever.
Close the valve and watersupply. Very small leakge can be stopped by using adhesive and giving less pressure. Confirm the leakage have stopped. Otherwise entire galvanised pipe will have to change