when i first started writing lyrics years ago, stream of conscious writing was a great exercise. i wrote nonstop gibberish, random rhymes, or words until i filled up a page or two from there i would then highlight anything that seemed interesting to expand upon - this can work in any writing field really...
i also love the notes app - any idea that comes into my head goes down there. great mini diary or thought collage to get the idea out whenever something comes to mind whenever wherever
as far as what to write about, that’s difficult to answer but i feel like these tools are a great entry point because its a casual low pressure way to get the juices flowing