Take it apart & replace the ball valve itself or the whole faucet with new one you can get parts online,hardware store
Hi Barbara: I think the directions from Cheryl may be good, but one thing I would add, if they don't say "Turn the water off and let the faucet drain first" is that when you take something off, have a towel on the counter and put the thing you took off on the upper left side of the towel, and the next thing that comes off, put that to the right of the first thing, and so on. When it comes to the part where you take the washer out, take it straight out, get a good look at it so you can see which end is up and which end is down. If there is a spring, make note of which end is up and down. If you can take a picture with your phone when you take each thing off, the better off you will be, but at least make a note about which end of the washer goes up, fat or skinny end, or whatever. Then, put it on the towel and proceed with taking off what needs to come off, and putting it on the towel. Now, take a nickle, penny, dime, or something, and put it where the washer goes on the towel. Take the washer to the hardware store and show it to them so that you get exactly the same type of washer. There are zillions! When you get home, just reverse the order of how you took things out. When you are putting the ball back into the faucet, there might be a little pin looking nub sticking out. That should match a notch in the fitting. Put it there and check to see that the faucet is facing the way you want it to before securing everything. You may have to wiggle it a bit. Turn the water on and make sure there aren't any leaks. You can do this, I did it, and I have faith in you:) I've also put in new faucets, and I'm not a plumber, just an old lady trying to save a few bucks. Good luck!
If you know what brand it is, you can buy a fix-it kit at a hardware store. It will have the parts necessary for most repairs.
Some manufacturers will send you free replacement parts, complete with instructions.