Before starting any plumbing repair, it's important to turn off the water supply to the toilet. The shut-off valve is usually located behind or beside the toilet.
Flush the toilet and hold down the handle until most of the water has drained from the tank and bowl.
Check all the parts of the toilet to determine where the leak is coming from. Common causes of toilet leaks include a worn-out flapper, a faulty fill valve, or a cracked tank or bowl.
Once you've identified the source of the leak, you can replace the faulty part. Flappers and fill valves can be easily replaced with new ones available at hardware stores. However, if the tank or bowl is cracked, you will need to replace the entire toilet.
Once you've replaced the faulty part, reassemble the toilet and turn the water supply back on. Test the toilet to make sure the leak has been fixed.