D50 black spot of photos
Ok, step one - find an appropriate cleaning solution - be it an air blower, like a rocket blower, a pads, alcohol and a qtip, etc. Keep in mind that it's not necessarily true more expensive means better cleaning... but I wouldn't go the dirt cheap route of alcohol and a qtip, though many claim they use it. Personally, I use a blower, but I havn't had anything more than dust thus far...
After that, prep a very clean environment, then take off your lens, and clean your mirror!!! Then when you think it's clean, try your camera again... if you still have spots, then you're going to have to clean the sensor... again, I'd use a blower, especially considering that if you ruin the sensor, you've pretty much ruined your entire camera.
If you STILL have spots, check your lens for dust, both front and back lenses. Still spots? Try another lense entirely (might be scratches). STILL on there? Then you probably have a bad sensor... see warranty information and pray :).
If any of this makes you queasy, look in your phonebook for stores that will offer repair, and try to locate the Nikon-qualified if possible.