Like a book, everyone has one in them. And they're easier to write than books...

Songs can be about things that matter to you, strong feelings you have (happiness, sadness, anger, whatever), historical events, people you know, random psychotic things... they needn't make a whole lot of sense, and you can claim some kind of level of artistic poetry if they don't.

You could start with the music, the words/lyrics, or the melody (try humming some random things, see what sounds catchy).

Who knows, you may even be able to sell your song to a pop starlet and make millions...