When writing a song, where do you start? Well, you can start with the lyrics, or with the music. You can start at the beginning of the first verse. You can start with the title, the hook, and the chorus. You can start at the end and work backwards.
Put simply, you can start anywhere. If you're stuck trying to think of a first line, then don't start with the first line. If you can't think of a hook, then don't start with the hook. Just skip over any place where you're stuck, and work somewhere else. Trust that everything will fall into place along the way.
Start with whatever you have already. Maybe it's an idea that you want people to think about. Maybe it's a fun little piece of wordplay. Maybe it's a chord progression that you like. Work with whatever inspiration prompted you to write, and build outward from there.