I am using D90 with programe Auto ISO 400, in

additionally some remarks: I would recommend the ISO 200for daylight, as the D90 doesn't have a native 100 ISO (LOW 1). For a beginner enabling auto ISO is the better choice: this way the camera adapts to different conditions automatically, and you can never forgat to change back from ISO 1600 from the night before. If your images are dark it has got nothing to do with the ISO (ISO 400 can produce nearly perfect images in daylight). Some things to consider:

• is the camera in the right mode (auto, P,..) if the main dial is on M, or sometimes also A, your exposure might be wrong.
• reset the camera (the 2 green dotted buttons next to the top screen,) should erase any setting you ight heve forgotten
• dit you shoot direct into the sunlight? also creates dark shadowed pics but this is normal

ISO400 is too high of a sensitivity to use in full day light. Set the ISO outdoors to ISO100 and learn how to use your exposure compensation. The meter in the camera is easily fooled in bright contrasty conditions, so if you are getting consistantly darker shots outdoors increase the exposure compensation a stop or two. Another way is to learn how to use the camera in manual mode and you find you will have better control over the exposure.