Spend a few minutes just listening to the song. Try not to focus on any one aspect of the song. Instead, let it flow into your ears and relax, allowing its melody and harmony to lodge in your memory.
Take a seat at the piano and play the first few bars of the song. Depending on how long it is, you're probably best off playing just the first verse.
Figure out what key the song is in. The easiest way to do this is simply to find the lowest note of the starting chord. Most pop, rock and jazz songs start on the tonic or main chord of the key.
Know that all the song's chords will be in its beginning key, unless there's a key change, which most songs don't have until the end.
Follow along with the music and play the bottom note of each chord on the piano. Together these notes form the bass line, which establishes the harmonic root of each of the song's chords.
Stack notes on top of the bass line by deciding if the chord you're hearing is major or minor. Major chords like C-E-G sound happy and bright, while minor chords like A-C-E sound sad and gloomy.
Sing along with the melody and plunk out its notes until you've got it down. It might be easier to leave out some of the melody's grace notes and ornamentations until you've got its basics down.