The answer to 'my shut-off valve is leaking' is that the most common causes are worn washers, corrosion, or a damaged valve seat. This can result in water dripping or spraying from the valve when it is turned on. The good news is that this is usually a relatively easy DIY repair, but you may want to call a professional plumber if you're not comfortable working with plumbing fixtures.
To fix a leaky shut-off valve, you'll first need to turn off the water supply to that valve. Then disassemble the valve, clean the interior, and replace any worn washers or valve components. If the valve is severely corroded or damaged, you may need to replace the entire valve. Be sure to turn the water back on slowly and check for any remaining leaks.