Check the back of the watch to see what it looks like.
If it has a series of 6 notches around the perimeter, then you need a spanner wrench. You can buy them lots of places online or on ebay, also at Harbor Freight stores.
If it has little screws, then you need a little screwdriver.
If it has a whole bunch of little grooves around the periimeter, then you need a special wrench (Rolex watches are like this.)
If the back is completely smooth, then it probably pries off, such as with a knifeblade on the edge. (Just don't cut your fingers in the process.)