once again Google to the rescue-- GOOGLE " how to add sink shut off valve" how to videos.
This process is too in-depth for me to just sit and type here for you without knowing if you are using Copper, PVC, or Pex. Plumbing. However it is a fairly simple process for the average handyman such as myself who has done it many times in the past. But for a novice I am suggesting DIY How To Videos.
For Copper pipes:
shut off the main water supply to the water-line you will be working on
turn the water faucett on in the kitchen and bathroom and leave it there until you finish working.
If you are working on the cold water line then only open the cold water from the kitchen and bathroom. This is to allow air into the pipes so that the water will drain when you cut the pipe-- water in the pipe will not allow you to solder (sweat) the pipes together-- the pipes must be clear of water to reach its proper heat range for the solder to melt and seal on copper pipes.
if the tee on your valve is 2 inches from outter edge to outter edge then you will want to cut a section of pipe of about 1. 1/4 inches long.
once the pipe is cut use emery cloth, sandpaper, or a wire brush to clean the pipe ends in and out before applying flux -- once you applied the flux-- plact the valve in place in between both pipe pieces and begin to heat testing periodically with the tip of a solder until you see the solder starting to melt.
With the water main shut of valve of course
The washer in the shut off valve has probably split or come off completely over time.
If the shut off valve turns but does not shut the water off, the washer in it has split and probably made it to the faucet. It was blocking part of the hole on the hot side and at some point either shifted or made it through the faucet and in in the screen in the end of the spout. Shut the main water off and replace the washer in the shut off valve.
Additional shut off valve, below sink or in basement. Incomplete pipes not tied into bathroom sink line.
Bring them through the wall or floor and put a shut off valve on them.
If the valve is a multi turn valve, it is because the valve seat is no longer good. Replace the valve. It it is a quarter turn valve, it is because the mechanism that turns the ball is no good and the valve does not turn completely shut. once again, replace the valve.
This depends what shut off valve is in question here. Where the leak is and what type of material is the shut off valve made of.
the hot water valve shut off valve shuts off the hot water. The cold water valve shuts off the cold water. ;()
Filter plugged, some have a shut off valve as part of the filter, mineral build up at the shut off valve in the supply line or in older pipes the pipe itself can be filled with minerals.
A valve in a plumbing line that closes the line before the fixture it is connected to. For water there are usually shut off valves under the sink and right above the floor for the toilet. For gas, there is usually a valve that a flexible line attaches to and then to the appliance.
What to use to solder a shut off valve
An air-intake engine shut-off valve regulates air-flow to the motor, and operates as an emergency shut-off valve. There are also shut-off valves on water hoses, so shut-off valves really control the inflow of any liquid or gas.