Clean the disc.
Clean the laser lens.
For lens cleaning use a lint-free cotton swab and as high percentage Isopropyl Alcohol as you can find. *NOT* 'rubbing' alcohol. Most drugstores will have 91% Isopropyl Alcohol (Make sure the other 9% is water and not other additives) Dip one end into the alcohol and swab the glass lens in a circular motion. Use light pressure, the lens will move around a bit when you do this. Don't worry, but avoid pressing too hard in any direction to be safe.
I use the same alcohol for cleaning CD's sometimes. Use a lint free cloth and remember to wipe CD's radially when you clean them (inside out or outside in)... never in a spiral around the disc.