Girlfriend wants to be dominated

My girlfriend has expressed interest in being in a full time Dom/sub relationship. We've done simple things in the bedroom like tying her up, spanking, things like that, but we've always been vanilla outside of the bedroom. I'm interested in doing this for her but being inexperienced I'm not sure where to start. Is there any blog or website, or even just specific advice anyone can give me to help? We don't want to go all out right away, we both know we need to start small, but are unsure where to start.