How To Write A Song

Step 2 Inspiration

This is often the hardest part: what to write a song about? I think you're best off finding subjects that you truly know about, and care about; something you're feeling in your gut. Otherwise, your song comes straight out of your head, and some of the worst songs in history were flawlessly executed cerebral exercises; gutless wonders.
So, the "big breakup" is a no-brainer; you're emotionally crushed and have a deep well of torment to dip your songwriting bucket into. If, however, you haven't just been dumped or you aren't unrequited or generally grouchy about anything, you have to look elsewhere. I've gone WAY back to my high school days, or used a friend's story, or used books or movies that impressed me. Watch out for "protest songs" unless you are truly angry or moved and have something new to say; otherwise they end up sounding whiny.
(Forgive me as I inject my opinion about whiny protest songs: "Excuse Me Mister" by Ben Harper: GREAT groove, great vibe, great attitude, but no beef: it's blaming the "other guy" for everything, and it comes off as whiny to me. In contrast: "Play the Greed" by Dar Williams: practical advice, saying something new, taking responsibility, no whining. Definitely a "cerebral" song, but she believes it enough to lend it guts!)

Don't write if you're not thinking about anything. Or at least, don't get attached to what you're coming up with. Wait for the inspiration, or go get it somewhere; meet people, watch an old movie, eat Doritos. Then write.