If you want to make a connection between some copper pipes and PVC pipes, then you need to have a special type of fitting which keeps the two pipes separate. It used to be necessary in the past to divide two pipes made of different metal from each other, as they could form an electric charge, but most of the time, the reason that you need a connection between your copper pipe and PVC pipes is that they just don't fit each other.
Step 1 - Add the Valve
Once you have turned off the water supply, cut off your copper pipe where it is about to meet the PVC. Make sure that the pipes are dry inside before proceeding. You may need to sand the copper lightly to smooth it down. Push the adapter valve into the copper pipe.
Step 2 - Solder the Pipe
Take your solder and add a drop to the copper pipe. Fix the adapter into the copper pipe. Push a female connection into the valve and turn it towards the PVC pipe. You should then cover the end of the PVC pipe in PVC cement and push it into the connection. Leave to dry.