If you have insomnia because you are worried about stuff and your brain won't shut off when you go to sleep, get a notebook and start writing down everything that is bothering you. Then tell yourself you will think about how to solve the issue when you wake up. I've also listened either to music that calms me down, or to an audio book on tape. Even though I am older than most, I've actually listened to the Boxcar Children's books on tape. Listening to those stories reminds me of when I was in school and during the lunch hour, our teacher would read to us. It's very soothing. Over-the-counter sleep aids can also help. Also, drinking warm milk with a touch of honey in it will help. Don't worry about falling asleep - just focus on centering yourself, listen to your breathing and relaxing. Sleep will come! Good luck!
Do you have it a lot ? If so talk to a doctor. If its something that just started then I'm guessing you are stressed about something. Exercise more! Consume less caffeine and sugar. If you still have problems, try reading. It always works for me.