Don't try and force yourself to write songs.

I find that when you think, "Okay, today I'm going to sit down and write a song," it usually doesn't work or it may not come out to be the best song. Usually, you will be inspired by an experience you've had or just how you are feeling. Let the music come to you. The melody will come into your head, or maybe you'll think of a really good lyric idea. It usually happens when you least expect it to. Just relax and live your life, then you'll sing about it when your heart and mind are open to creativity and expression.