This website has a lot of information and can probably help familiarize you and your GF/future Mistress with some of the ways to transition your current realtionship into a female led and dominated relationship.
It might be challenging (especially if you’re naturally submissive) to just start talking about her taking charge, but trust and communication will be essential in your future and so, difficult as it is, now is the time to start discussing with her where you see your relationship going. Good luck!