Notate your melody on blank sheet music paper, complete with rhythm and lyrics written underneath. The melody is the basis for your song and will guide your piano accompaniment.
Determine the key of your song. By taking note of the sharps and flats in your melody, you can determine what musical key it is in. The key of the song will tell you which chords are appropriate for the song and which are not.
Choose chords for your melody. Using a chord generator like the one at Zebrakeys.com, you can learn the basic chord shapes for chords in your song's key. Use your personal taste and judgement to determine which chords will sound good in your song.
Notate the chords on your sheet music paper. Do this by writing the letter name and quality of the chord above the previously written melody and lyrics. As the chords change, write the new names of chords above the corresponding note in the melody.