Don't make a big deal out of it. If you don't freak out neither will she. Just explain that is a normal thing that happens to all girls. Tell her that each girl starts when her body is ready. As long as you reassure her that it is normal she will be able to deal with it.
I had friends who started at age 8 and others who didn't start till they were 16. As long as a girl understands it is normal, she will handle it just as well as the next girl.
As for the sex talk, don't make it contingent on starting a period. It should be something that is discussed when she is ready and you are comfortable. I didn't get the sex talk until 2 years after my period but my sister got the talk before she ever started hers. There is no right or wrong, just talk when she is ready.