Okay. First of all, you need to remember they're just dreams. I know that's easier said than done, but sometimes they can seem really real and that's scary, even for a grown-up.
Second of all, you need to tell Anak Vader, okay? I don't know if he can help, but he needs to know about them. And if you can't tell him, tell your mom. It's important, Luke, okay?
And, if you want to talk to someone besides them about anything - dreams or anything else - I'm here. I know you don't know me, but you can trust me and I'll do anything I can to help you. I'll give you my word on that.