On the D50, do the following:
1. Make sure the battery is fully-charged.
2. In the menu, select 'Mirror lock up'.
3. Remove the lens.
4. With a blower (the rubber bulb part BUT WITH THE BRUSH REMOVED), blow the sensor. The lens opening should be facing down. Be careful not to allow the blower to touch the sensor.
5. Replace the lens.
6. Switch the camera off - the mirror will drop to its normal position.
Dust spots are a fact of life with DSLRs. I blow my sensor about once a month.
If that does not work, there are kits for wet and dry sensor cleaning, but they need to be used with care.
Alternatively, any Nikon centre will clean your sensor for free if the camera is still under warranty. If not under warranty, the charge is about 20 euro. I use the one in Paris and they do a really good job.
Hope this helps.
Nikon D700, Nikkor 80-400, Fuji X Pro 1, Fujinon 35 f/1.4, Fujinon 18 f/2