You might find yourself needing to work on your plumbing sometime, specifically replacing an angle stop. If your angle stop is dripping, you can try to slightly tighten the nut right under the handle but if that doesn’t work, you are likely looking at replacing it.
You will need to turn off the water to the house. Some angle stops have a water supply line connected to them (like a toilet angle valve), but most are separate from the supply line. You have to be prepared to drain the water from the line when you start removing it. It’s easier if you just point the supply line into a bucket, but be prepared for water to come out once you start working on it.
The easiest replacement is on a bare copper pipe. Use two wrenches and hold the nut with one and the valve with the other. Start unscrewing it and it will come out easily. I generally just push the new valve on the pipe without replacing the ferrule or nut. The nut will squeeze the ferrule into any voids and close off the leaks. Again use two wrenches, one to orient the valve and the other to tighten the nut that holds it in place.