A part of getting to sleep is understanding what is keeping you awake. If you drink a lot of caffeine shortly before bed you probably won't get to sleep as quickly as the person who does not drink caffeine. Also, certain medications, such as those prescribed for ADHD, can make if difficult to get to sleep. There are a few things you can do though that will help.
First, try not to drink caffeinated beverages before bedtime. Second you can try some warm milk, which releases tryptophan, a chemical that will make you drowsy. You can also try melatonin, a pill you can buy at any natural health store. Melatonin is a naturally occuring chemical in your body that makes you sleep. If all else fails, talk to your doctor about a sleep aid.
I'm have historically been a "bad sleeper" my entire life & these are a few of the things that helped: Try working out a few hours before bed, not just before otherwise you'll be up all night. No alcohol, not even wine. Switch to decaf, painful but worth it. Consider Blackout shades. Consider meditation, it helps clear the mind.
Slot of thing can keep you from falling asleep easier. It could be hormones, poor sleep hygien (tv, pet in bed, snoring partner, ect...) or physical or stress. Go to sleepeducation.com and start educating yourself on sleep. One quick trick you can play on you body is to take a hot shower before bed. Your body temp has to drop a little before you can fall asleep. The cool down after a hot shower fools your body.
Some of these other answers listed here are right on the money and some are not. In the end, you may just news to go see a sleep specialist to determine any underlying cause of your insomnia. Is is important though to find the best treatment for yourself.