What can I do to get rid of the dust on my viewfinder?

I own a Nikon D60. I take really good care of my camera, but I went I go to use it recently I found I had a black piece of lint on my viewfinder. It does not show up when I take a pic, but its quite annoying. I have no idea how it got there. I have not used my camera for about 4 months and it has been sitting in safe place. So since I'm a hobby photographer and have no idea about this how did the dust get in there and how can I get rid of it? I tried a lens brush and a q-tip and its still there. Will it affect the camera? It doesn't affect the pictures, but its annoying and distracting. If i took it somewhere how much will this cost approx?
After months of not taking pictures I really want to get back into it, and picking up my camera and finding this kind of upset me a little.

I don't know what you mean by "on my viewfinder" so I will assume that it means that you can see the object through the viewfinder but it's not part of the photos. I think it's either on the mirror or the focusing screen. The focusing screen is right above the mirror. You might be able to blow it off with canned air. When you use canned air make sure you keep the can upright otherwise it may allow a liquid to spray out.