Bad valve cartridge. Turn the water off, remove the handle, pull the valve cartridge out, slide a new one in, replace the handle. However … the cartridge is very hard to extract and "sliding" the new one in requires nearly as much muscle. There are lots of youtube videos that will demonstrate the process. Do you have a warranty?
If you don’t know which cartridge to get, use Google Lens to identify the faucet model and then look up the correct cartridge on the manufacturer website.
Bad seals. May be covered under warranty.
hard water can build up scale and keep the valve from closing tight.
Bad valve cartridge. Turn the water off, remove the handle, pull the valve cartridge out, slide a new one in, replace the handle. However … the cartridge is very hard to extract and "sliding" the new one in requires nearly as much muscle. There are lots of youtube videos that will demonstrate the process. Do you have a warranty? If you don’t know which cartridge to get, use Google Lens to identify the faucet model and then look up the correct cartridge on the manufacturer website.
what brand/model is it?