First, just lay down in bed. Laying down in bed will induce you to sleep.
If that doesn't work, get up, walk around and stretch a little. This releases chemicals and can persuade you to sleep.
If this doesn't work, go downstairs and drink some tea, get relaxed before trying again.
Now if this doesn't work, you'd probably need to see a doctor about insomnia problems and get a melatonin prescription or something.