First, consider what topics there are you'd like to write about. Then take one of those topics and do a couple of rounds of freewriting, formulating all of the association you make with that topic. Now pick out the sentences that draw your attention. Write those down and put them in a new order. Either do more freewriting or start puzzling to make a story. Listen to music and analyse how the lyrics are structured. Look up rhyming schemes and song structures. Or don't do any of that and do your own thing.
Then find a beat or a melody and start vocal jamming with the words and sentences you wrote down. Make it flow. Go back and forth between the melody and the lyrics, until you are satisfied with the result.