Before you tell your parents....
1) How are your grades since you started? If you are getting good grades, and have since you started, no problem. Tell them after you get a good report card. It's harder for them to argue against a good report card.
2) get together some scientific studies- Current and former marijuana use: preliminary findings of a longitudinal study of effects on IQ in young adults -- Fried et al. 166 (7): 887 -- Canadian Medical Association Journal (a real goodie, if you're a weekend toker! (or can convince your parents you are one-lol) Do some research- does anyone in your family suffer from any diseases like Alzheimer's, MS, osteoporosis, MS, ALS, cancer- run a search on cannabis, abstract, and the disease. If they bring up the "hormone changes" There is a study called "Effects of chronic marijuana use on testosterone, luteinizing hormone, follicle stimulating hormone, prolactin and cortisol in men and women" (sorry, don't have the URL, but the title should enough to find it on google)
3) Prepare to present your arguments in a calm and adult manner. Even if THEY blow up, keep calm! They are likely NOT going to like the news that their baby-boy does drugs. They may bring up the "gateway theory", but there has been several studies recently that disspell that myth. Again, google it.

Good luck- Granny:stoned: