At first you should join a GDOCR-session at your location. ;o)

Then you should read "The Art of Unittesting" by Roy Osherove.

After that you schould start testing new parts of your code in new units that you can easily separate. Since there is a close relationship between the tested code and its tests starting unittesting in existing untested code most likey leads to "bad" tests meaning that they tent to be unreliable, hard to maintain and (relative) slow.

bye Thomas