Easiest would be Self Closing Hinges. You just replace the existing hinges with one or two of those. All you need is the size/shape and they screw right into the exact same spot. You don't even need to take off the door, just one hinge at a time.
https://www.amazon.com/s/ref=sr_nr_...941120011http://www.gtdoorsandlocks.com/1689/
http://www.gtdoorsandlocks.com/1678/?gclid=CJfeqpvTldQCFRxWDQodLY8N9A
The door will still slam with self closing hinges. The only options are a hydraulic door closer or installing door silencers. Door silencers are used for metal doors. If you have a metal door frame you will see little holes along the surface where the door hits the frame. Those are for the silencers. You should be able to get them online. They are just rubber grommets that fit into those holes if not you can glue them on. door banging will no longer be a problem.