I have an eating disorder and I've never told anyone
Why have you never told anyone about your eating disorder?
I heard it's no use telling anyone, anyway.
I understand being nervous or embarrassed to tell people when you are struggling with something like an eating disorder. It can be good to let close friends or family members know about the disorder, so they can help you deal with it.
Do you think I can tell my family about it?
Yes, I think that your family should be the first to know.  They might be upset at first, but they will try their best to get you help along the way.  No matter what, you will need their support as you fight this.