I didn't tell my parents for a couple of years. I don't really know why... When I did, I just told my mum "you know how I've been a bit depressed... well i've been diagnosed as bipolar" answered her questions how do you get it... "no its not yr fault" what can you do "drugs" etc for about 2 mins then changed the topic and haven't talked about it since.
It really does depend on what you want from them, what kind of response are hoping for / dreading.
Good luck