Every faucet and toilet in a home should have its own stop valve so you can easily make repairs. Installing one is usually not difficult, though you may find yourself working in cramped quarters.
Take care to buy the correct valve. Its inlet must accommodate the size of pipe coming from the wall (usually 1/2 inch but sometimes 3/4 or 3/8 inch). Its threaded spout must match the size of the supply tube -- either 1/2 or 3/8 inch. You can install a compression or a sweatable valve.
If the pipes are plastic, simply cut with a close-work hacksaw and cement the valve in place.
Some water will remain in the pipes and tubes after water has stopped flowing out of the faucet, so place a bucket and towel underneath.
One or two hours
Tubing cutter or close-work hacksaw, combination wire brush, two adjustable wrenches, flux brush, propane torch, fiber shield
Cutting and joining copper pipe
Shut off the water; drain the line; place a bucket or towel to catch debris
New compression or sweatable stop valve, flux, solder, possibly a new supply tube