When you don’t know how to start writing your own song, the first thing to do is just play any instrument and keep humming melodies. By humming, you will get to discover a certain melody that will give rise to your composition. There are musically gifted song writers that can write songs instantly while travelling or even in the crowd. But for you, still amateur in this field, you can do it in silence. Find a place where you can concentrate on the melody. Remember that the more silent it is, the more focused you are on the melody that you would like to create.
When you already have a melody, you can start writing lyrics to fit into it. The details depend on you. You may want to try writing about your past or present experience. It’s up to you. Be creative in this matter. And lastly, don’t forget to bring with you always a recording device for you to record what you have composed. You may forget about it in a few minutes believe me.