Is the screen totally unresponsive ?
if so, try to reset the iPhone (press and hold the home button and the sleep button at the same time until the iPhone restarts (should take not longer than 10 seconds)
if it's "only" the top area - the digitizer is most likely defective.
since the phone isn't to old and should still be covered by warranty - i would go to apple and let them fix it.
if you voided the warranty - you would most likely have to repair it on your own.
you would need the front panel kit wich includes the front screen and the needed tools.
use the front panel guide for that operation