Conservative, resistant minds are an uphill battle. But I wholeheartedly believe it's a hill you should climb nonetheless.

Start by encouraging your dad to read the unbiased marijuana info at this site: http://www.erowid.org/. It's very thorough, impressively objective information about the effects of weed and other psychoactive substances.
Also present him with select info from http://www.norml.org/, which is also a good source.

Don't expect to change his closed mind. But take pride in the fact that you may make a slight inroad into its surface. And be proud of the fact that you're learning to establish and argue a valid, important point about the benefits of this substance. You'll get better at these arguments. And more persuasive. And you will be contributing to a good and necessary effort. And maybe just maybe you'll eventually begin changing other minds that are less resistant and more influential.