How to get rid of a speck of dust on my camera?

I have a DSLR, Nikon D5000. I don't know if what is showing up on pictures is a speck of dust on the mirror, or a dirty sensor, or what. I can see the speck of dust in the eyepiece. It gets bigger on the picture if I zoom in and it kind of appears as a mole. It's on the same place in every photo. What do i do?