When you write a song, make sure that it's from the heart. If you don't know where to start, write down how you feel right then and there.
Example:
I thought that you'd be the one for me. Then you told me goodbye. Now, here I am, sitting on the bathroom floor, eyes full of tears. My heart is in a million more pieces than what is was before you came along.
Anyways, a song should always come from the heart. You don't have to look at it like writing an essay so nothing can be repetitive. Just write whatever you want.