Writing music is like writing poetry. Use your imagination. Any of your past-life experiences can help you. Listen to some of your favorite bands/artists to get some ideas. Look up the meanings of their lyrics, to help you determine a meaning for your song. Write about being heart-broken, and how that affected you and your life; but don't be like Taylor Swift, writing relationship songs 24/7. It gets very annoying. When you write music, don't be repetitive. The only things you should repeat, are the choruses, because they're the main part of any song. I'm not a fan of One Direction at all, but about every girl likes them, so one of their choruses from one of their famous songs, would be, " da, da, da, da, that's what makes you beautiful." Writing a song is like telling a story in the lyrics, and being a part of that story. It all helps you focus on what you're writing.
If you want to write a song about society or bullying, then you can look up some of Linkin Park's lyrics for some ideas. Another great band is 30 Seconds To Mars, as well as Rise Against. You can use some of your favorite bands to inspire and motivate you.