I usually start by just playing four notes over and over then I just hum along. It is really effective to form a base of the song.

Then I start thinking about how I am feeling at the moment. It can be sad, happy, jealous, lonely, worried, grossed out, etc. It can really be about anything you want. You just wanna make it sound nice as a whole.

Then you start to think of lyrics. You need to show how you feel in your lyrics. It needs to feel relatable to others and they will listen to it more. Of course creativity is key. Trying to stand out is the best. It will cause you a lot of success.