Keep it simple with your song, most songs are based on four chords, and often the same four chords. The video at the end demonstrates that.
The difference between the songs is mainly the melody and the rhythm
Once you have your chord structure and a rhythm you like, you can start on you melody. You can either hum over the top a tune that may fit well or have a friend try to improvise along with the chords. You can even use apps such as Loopy HD http://loopyapp.com/