The answer to 'how to fix shut-off valve' is to first locate the shut-off valve, which is typically under a sink, behind a toilet, or in the basement near the water meter. Once you've found the valve, turn it clockwise to shut off the water supply. Next, use a wrench to loosen the packing nut around the valve stem, and then remove the valve stem. Inspect the washer or seal inside and replace it if it's worn or damaged. Finally, reassemble the valve, turn the water back on, and check for any leaks.
If the shut-off valve is stuck or won't turn, or if you're not comfortable performing this repair, it's best to call a professional plumber to handle the job.