Perhaps you need to be more specific to your situation.
Like Majesty said, the power button is on top right of the phone.
However, if you still couldn't turn it on with that button. It means that your phone has very low battery. Try charging it for a few minute before you attempt to turn it on again.
If after a while it still didn't turn on, then try hold the power button and the round button together for about 10 seconds. The apple logo should appear.
if all else fail, try the genius bar at the apple store.