I think that everyone handles telling their parents in a different way. I found it very difficult to tell my mum. I just started crying and blurted out 'I'm pregnant' on the phone. She was very very supportive. I thought she was going to get really angry but she wasn't, she was fine, and responded way better than I could have ever expected. If you tell your parents in person or over the phone, they might respect you as that is something very difficult to do. You could write a list of all the things you want to say to them and memorise it before you talk to them.
However, I also think that writing a letter would also have its own benefits. You would be able to express the situation clearly and tell them everything you want to without interruption. You might be able to portray yourself in a more mature and responsible way in a letter. It really does depend on your parents' personalities. The best thing is just to get it over and done with. It feels like a huge weight has been lifted from you and it's very difficult to have this on your mind. Also, the sooner you tell them, the easier it will be as it won't seem to them like you've been keeping it from them for ages. Good luck x