I think it's important to note that most people know what BDSM is. And most likely your girlfriend is aware of its existence as well. But you're going to have to judge her reactions to things, no one on the internet can understand those.. only you. So use some tact and figure it out and bring it about slowly like sweetly_wicked said. Just be aware that she may be bedroom-only. On the plus side, I think that girls generally like a guy who is open-minded to things sexually.