Hey there, first off I'd like to say that you are very strong for even considering telling your parents about feeling deppressed lately. Getting help is often the hardest step towards recovery, but it is also the most important.
Finding a calm time when your parents aren't busy is important, or if it's hard to catch them not busy tell them that you have something really important you want to talk to them about. Try writing a list of all the things you want to tell them such as why you think you're depressed, how you think you could get better with support, and how you'd like their support. Some things, such as the drugs and self-harm, will be hard for you to say and also for your parents to hear. Some parents have a hard time accepting these things when they find them out, but they'll come to terms with it. Keep in mind that if they have a bad first reaction it won't last forever. They should come around and support your healing process in the end.
I'm not a parent, or anywhere close to being a parent but I am a big sister. I can tell you that if any of my brothers came to me and told me that they have been depressed I would do everything in my power to support them, get them in therapy, and let them know that my door is open 24/7 for them to talk to me.
I hope that this helped and I wish you the best of luck
