Good luck with it all. I remember my mum had 'this' talk with me, only thing was that she was too late, I had already been getting my periods for half a year, I just hadn't told her. I didn't know how to break the news to her when she decided to have the talk so the next month when I got my period I pretented that was the first time

So maybe you should ask her if she has had one yet and how much she knows about it all first - it may save some embarrassment. Also I have noticed a Kas Cooke book out ther called 'girls stuff' (I haven't read it though so don't know how good it is) - maybe something like that would be useful, teenage (and pre-teen) girls don't always like talking to their parents about things, but it is good for them to have all the useful information.