How do I fix a leak that is just behind the hot water shut off valve beneath my bathroom sink?

 

Shut off the water at the hot water heater, or the water coming into the house. Now you can remove the shut-off valve (or angle stop) under the sink that is leaking. What it will take to fix it will be determined by what kind of pipe and connection you are dealing with. If it's galvanized iron or copper with a thread adapter, and the pipe itself is in good shape, replace the shut-off valve with on just like it (same threads on both the incoming and out-going side). However, if the shut-off valve is attached to copper pipe with just a compression ferrule and nut, fixing it may be as simple as just having to tighten it.