as a couple of people have said, you need to get your dad to understand about cannabis and the difference between Soft recreational drugs ie: cannabis and Hard unsociable drugs ie: Cocain/Heroin etc...
get him to sit down with you at your pc and show him some of the information that is available on various websites about cannabis (well chosen in advance)
if your dad has an ounce (no pun intended) of respect for you he will sit down and atleast listen to your side of the argument.
failing that......... move out and get your own apartment then he wont know what you do.