It'll be a dust spot on your sensor. They tend to show up most at smaller apertures. Try stopping down to f/22 and photographing a plain white wall or sheet of paper to see how bad the situation really is.
You can get paranoid about it though - dust is a fact of life in cameras and as fast as you clean the sensor more gets on there. For the odd spot like this one, I'd just touch it out in processing.
If you want to clean the sensor there are numerous methods such as Rocket Blowers,. which I don't like much as they tend to just distribute it elsewhere - static dry brushes, and several wet cleaning solutions on the market so it's personal preference as to which method you choose.
Do a forum search on sensor cleaning for more info and walk through instructions.