If you have a gas supply then you will have a gas meter with a shut off valve.
The gas meter will usually be located either on the boundary or somewhere on the outside of your house.
The shut off valve will be on the inlet side of the meter (usually on the pipe coming up out of the ground).
If the handle of the valve is in line with the pipe then the gas is switched on.
To turn the gas off you will need to turn the handle so that it goes across the pipe (you may need a spanner to do this). If the valve is really jammed we may be able to help, or you could contact your gas supplier and request their assstance.
If you are having trouble locating this shut off valve please feel free to contact us.