In my opinion, great sex is about communication, and rough sex clearly needs a lot of this! Next time you have sex, when you're all cuddling in the afterglow, maybe try and bring it up, ask her what she had in mind, let her know you're willing and excited to try, and I'm sure she'll be very pleased by your enthusiasm. And there are so many levels of roughness, she might just be looking for a light spank and you holding her down, or she might want to be tied up and spanked hard, or any of a million different combinations.
So, if you want to start fooling around with rough stuff before getting into a full on discussion, try a light smack on the ass or lightly hold her down and see how she reacts... take your lead from that and have fun
"You will always be loved, and you will always be in love with love... Don't be afraid. There are exquisite things in store for you. This is merely the beginning." - The Picture of Dorian Gray - Oscar Wilde.