I'm transgender, and I need help on telling my mother?

Okay, uh, I'm 16 and FtM, and I've known it for a while now. I'm just not sure how to bring this up to her. It's not that I'd think she'd have a problem with it, but I'm worried she won't understand. She's made hints and I think she thinks I'm a lesbian, or wouldn't be surprised if I was. One time, and I don't even remember how this was brought up, but she said something like she could see me being the more 'butch' one of the two. And in my head, I was thinking, 'Come on, just take it one step further.' And I know that probably would have been the perfect time to tell her, but I just wasn't ready. Anyway, she's open minded, but we've never once discussed the concept of transgender. And I can't bring i up hypothetically without making it obvious. So, should I just come out and tell her? And how does one bring that up in conversation? I don't want to make a big deal out of it, because I don't even really want to talk about it.