The first step in creating a piano song is choosing which key you'd like your song to be in. Play around with the various major scales until you find the one you'd like to write in.
A good place to start the pattern of chords is the root chord. This is formed by playing the first, third and fifth notes of the major scale. Play these notes simultaneously with the left hand.
Next, a common place to go is called the "vi" chord, formed by the sixth, first and third scale degree, in ascending order. (If this were C major, you'd play an A, the C above that and then the E above that.) Try this chord. Also frequently used in this position is the III 7 chord, formed by playing the third, seventh and second scale degree in that order.
Next might be a ii chord, formed by the second, fourth and sixth scale degrees. Or try the IV chord, formed by the fourth, sixth, and first scale degrees.
Most simple songs will have, as their fourth chord, the V 7 chord, which is formed by the fifth, seventh, second, and fourth notes in the major scale of your key, in ascending order.
Next, create a melody by playing notes in your key's major scale in the right hand. Play around with it until you find a pattern you like. Then play this melody over your repeated chord pattern, until you find a combination you like.