I don't usually decide in advance what to write a song about. I'll start to sing a tune and play a chord progression or accompaniment, and the song will become a certain shape, have a certain feel. That's when ideas for lyrics start to pop into my head.
But if I had to offer any advice it would be this: use original detail. Your song doesn't have to be a true story or be autobiographical, but if it's made up of original details from your own experience it will feel much more authentic. For example, you might be writing a song about being lost at sea, draw from your experiences of being on the water - how the boat moves, how it makes you feel, what the expanse of water looks like, how the horizon curves etc.