Either:
# You forgot to take the lens cap off ;)
# You are using the camera in S (shutter speed priority) mode with a very fast shutter speed, a low ISO, or both;
# You are using the camera in M (manual) mode with a fast shutter speed, a small aperture, and/or a low ISO;
# Your camera has a problem.
The solutions are:
# Before taking any photos, take the lens cap off and put it in your back pocket;
# Rotate the mode dial (the large rotary dial on the top deck of the camera) to select '''AUTO''' mode;
# See #2 above;
# Have the camera checked by a knowledgeable repairperson, ask an experienced photographer for advice.