Plumbing – Green Corrosion on new sink shut off valve after only a year

You need to find the main shut off valve in the road, side walk or somewhere on or near your property. They are usually placed in mini manholes with easy access:

But if you're unlucky you will need to get some building plans and try to find where on the plans the supply pipe runs and find the valve at the connection point to the main supply. In most cases it will be near the connection point of the main supply:

but if its not, or if you struggle to close it call your council to help you. You don't want to break that!

NB

It is easy to snap these valves- if its rusted badly apply (known as WD-40, LN-40, HT-40) spray oil based rust release (spray leave an hour try again.. repeat a few times), TAP the valve with a hammer (!TAP!, do not whack it, to help release the valve. Also, usually opening the valve slightly helps to release it then try to close it.

I think what is leaking is the drain. Does it look like the object sticking out to the side in the below picture? If so, this is used to drain water from the house side of the supply when the valve is off.

It sounds like you over tightened it in the first place which started the leaking. If you've broken only the cap, you might be able to find a replacement. If the stem and threads are damaged, long-term you will need to replace the entire valve. As a short-term fix, you just need to block it some how. Marine epoxy is handy for emergency plumbing repairs.