Either:

1. You forgot to take the lens cap off ;)
2. You are using the camera in S (shutter speed priority) mode with a very fast shutter speed, a low ISO, or both;
3. You are using the camera in M (manual) mode with a fast shutter speed, a small aperture, and/or a low ISO;
4. Your camera has a problem.

The solutions are:

1. Before taking any photos, take the lens cap off and put it in your back pocket;
2. Rotate the mode dial (the large rotary dial on the top deck of the camera) to select AUTO mode;
3. See #2 above;
4. Have the camera checked by a knowledgeable repairperson, ask an experienced photographer for advice.