How do I begin to discuss menstruation &what sex education Itell tomy 12 years old daughter.

There are many, many books that can really help open it up for you. Try something like this: What's Happening to My Body? Book for Girls : A Growing Up Guide for Parents and Daughters. It is available on Amazon for under $10.
You can use this to explain and discuss between the two of you. Hope that helps.

Your probably "a day late" as kids these days grow up very quickly. More than likely she has talked with kids at school and has some very wrong ideas on the issue. Start by telling her that everyone's body changes during adolescence into that of an adult. Tell her the changes, how they happen, what to expect, how it feels... ask her what she knows, thinks. This could be a time to correct any incorrect ideas. It may be uncomfortable but is extremely necessary. Kids get very wrong ideas and have major fears based on these ideas. Look online... there are probably sites that offer some explanations that would help. Good luck. Cathy

I totally agree with Cathy about being "a day late." My daughter grew up knowing about it. She started when she was 12 and had no problem coming to me the day she started.

Now, the best way to approach it since you have waited so long is to ask her if she knows about it and ask her to tell you what she has learned, as many of her friends have already started I am sure.

You need to assess what she already knows so that you can lay any myths to rest.