One of the most common challenges I hear about songwriting is not knowing where to begin. With the lyrics? With the music? Or something completely different? It’s a rather simple challenge to find a solution to, really.
Start with a single note or single word
Play a single random note and just listen to your imagination. Does a second note enter the picture? Does a chord or harmony enter the picture? If so, then add what you hear to the mix and keep listening like that. Don’t think about it or edit what you hear in your mind, just dictate.
If you don’t hear anything in your imagination, then force it. Play a single interval up or down. Does that spark another idea? If so, then dictate. If not, play a different interval. If after a number of tries your imagination doesn’t take over, then force it even more. Play a pattern or a sequence that you are familiar with and build a melody around that.
The important thing to remember here is that your melody is not going to be perfect. It’s just a first pass. You can edit after you have 32 bars or so of material to work with. The point is you can’t have a melody to edit if you don’t start writing one.
You can apply the same technique to lyric writing. Write a single word. Use a random word generator or just pick a word from a book. What does your imagination tell you to write next? If nothing comes to you (which would mean that you really just aren’t paying attention) force it by just making stuff up. Rhyme comes later. Prosody comes later. Form comes later. Same as with music, you can’t edit a lyric until you have a lot of lyrics to edit.
An exercise like this should open your mind to the constant stream of ideas that is always occurring beneath the surface of your conscious mind. Just practice opening the aperture until you always have music and lyrics to write.