If you are working with copper plumbing lines and will be soldering, there are steps to follow to get a leak free joint. Copper will last a long time, and so if you get a leak, it will usually be at a joint of a fitting.
The first step in soldering a fitting is to clean it. I don’t mean with soap and water, I mean to get the surface down to clean untouched copper.  Basically, you will be abrading the surface of the copper with a tool or with emory cloth. Emory cloth is like a thick flexible sandpaper that scratches the end of the copper pipe. In a pinch, you can use steel wool.
Tear off a piece of emory cloth and wrap it around the end of the copper pipe. Hold the strip around the end of the pipe and move it back-and-forth. This will remove any corrosion from the pipe and scratch it up so that the flux can flow into the joint once it is heated. You want the cleaned surface to look like a shiny new penny when done. You can also use copper cleaning tools that have openings with stiff wire bristles around them. They perform the same function.
To clean the interior of fittings, you can use a wire brush specifically for that purpose. They come in diameters that correspond to the fitting’s size. Once everything is shiny and clean, you are ready to flux the joint and apply the torch. By the way, once you have cleaned the copper, don’t touch it with your fingers. The oils from your fingertips could interfere with the flux and solder flowing.