How to delete songs off computer?

How do you delete songs from the computer or from windows media player(wmp), I connected my ubs cord with my phone connected to put songs on wmp so I clicked add to wmp playlist. After doin so the song would play but couldnt find the songs I recently added , on the side of the wmp it said last playlist played && it had the songs I wanted but then disappeared. Are the songs still on the computer I need help !!!!