try not to touch/clean the mirror of an SLR camera - its a silvered mirror and very prone to scratches.
the most you can do is to use a blower to remove the dust. if you are unlucky enough to have water spots, saliva or other liquid fall on the mirror, you can try to use isopropyl alcohol on a cotton tip to remove it - but do it very gently and don't try to clean the whole mirror, just the affected part.