Do what Bandcoach said.I practiced for a few weeks and now I can already recognize very simple melodies and chord progressions.
Here is a tip which helped me a lot.If you have a smartphone,download a free piano app.While you're relaxing, watching a movie or a TV show, pay attention to the music playing in the background and try to recognize some of the intervals you hear.Then pick up your phone and play the notes to see if you got it right.This way you're enjoying a movie and practicing at the same time.