Ever improvised? If so, have you ever written a song off of what you improvised? Or do you not even know what improvisation is? Well, to improvise is to compose, play, recite, or sing (verse, music, etc.) on the spur of the moment. It means, in an easy way, that you make up a song, a melody, and such. There is no set music at the time you make up the song, but you can write down the music you improvise, which is composing. Improvisation is hard for some people, and easy for others.
Improvising tips
Sometimes, if you can't improvise, try listening to another great classical composer. Try Bach, Beethoven, Chopin, Mozart, or Tchaikovsky. Maybe they'll give you inspiration.
More improvising tips
Maybe, if other music doesn't help you, try going outside and walking around. Perhaps something outside will help you. If not, then just walk around your house or the environment you're improvising at and look at the objects.