Take the bobbin out, and press the lever which releases the hook race cover. Now the hook will come out. The hook race is the metal lip or edge of the hook. Brush off any lint and put a couple drops of oil on that metal lip so that when the hook rotates the mating surfaces have lubrication.
Here, I found an illustration for you:
http://www.gloversvillesewingcenter....aintenance.htm