Admit that you were wrong to do that, right at the beginning of the conversation.
I went to Lagoon with my best friend and my Boyfriend. I wasn't supposed to see him. My mom just found out what happened. My dad doesn't know yet. I shouldn't have lied. Now I'm going to have to face my dad. What should I do or say?
Admit you were wrong immediately, apologize, be mature about it, promise it won't happen again. Do not complain about any punishment they might give you, tough it out and be more careful next time.