It may seem like an odd idea, connecting plastic--or PVC--piping to metal pipes, but it is often a necessity. Many older homes contain metal pipes, and if you plan to repair or add on to these types of existing plumbing systems, chances are you'd rather work with the much more flexible PVC than copper or galvanised steel. Fortunately, this procedure is both possible and relatively painless.
Clean the end of the PVC pipe you plan to connect to the metal piping with a damp rag. It doesn't need to be perfectly clean, just remove as much dirt as you can.
Apply a coating of PVC primer all around the outside of the PVC pipe and down about two inches from the edge of the pipe.
Wait for the primer to dry, then apply a coating of PVC glue over the primed area.
Insert the PVC pipe into the male connector piece and give it a quarter-turn to ensure a good bond between the pipe glue and the connector.
Wrap the threads of the male connector with Teflon tape.
Screw the male connector into the metal female connector.