When I'm writing, it's the weirdest thing: it's not even a conscious process. I'm not even thinking when I write, and then all of a sudden, I'll have a song that makes me feel so much better than I did before.
Write down everything you can think of, no matter how stupid it seems. I always write down my thoughts throughout the day. Sometimes good things come out of it, and I'll find an idea to develop into a song, so my best advice is to try and draw inspiration from everyday things.