A.F.
I'm not a mom to a pre-teen yet, but I sort of remember those times...
I would suggest starting the conversation just asking her what she knows, what she's been hearing at school/on tv/the web, etc. What are her friends talking about?
Just ask questions - let her lead the conversation. "I remember when I was your age my friends and I were starting to be interested in boys and boyfriends. Do any of your friends have boyfriends yet? (Wait for answer) What does it mean to her/you to have a boyfriend? Have any of your friends kissed a boy? What did she tell you about it?"
I truly believe that if you open a dialogue about **what she's dealing with right now** - and just ask her questions and get her to open up and share without judging she'll always feel comfortable coming to you when she needs to.
I really think the key is for you NOT to do the talking about sex and relationships - she will probably know a lot more than you think she does. Let her tell you where she is mentally and emotionally. Teach her through these conversations that you're here to support and help keep her safe and happy - not to judge.
You can use these conversations to figure out what her true feelings are - just by asking questions.
(Driving someplace in the car - just you two)
You: I noticed on The Show You Like that Joe kissed Maddy. Do you know anyone who has kissed a boy?
Her: Suzie said she frenched kissed a boy.
You: Oh? Did Suzie tell you what she meant by that?
Her: Yeah.... she said he stuck his tounge in her mouth...
You: What else did she say about it?
Her: That it was gross/That it was cool.
You: What do you think about french kissing? Do you think Suzie is old enough to do something like that? When do you think someone is old enough to do something like that? Why? (Wait for responses after each question.)
Then, you can add your own thoughts on the subject. Make sure she knows where you and your husband stand on the subject, but also make sure she knows that no matter what, she can talk to you about anything - even if she does something that you wish she wouldn't.
ie: You know, I think Suzie might be a little young (or immature) to be french kissing. I think that's something that should be reserved until she's older and has been dating a guy for a long time. It can easily lead to other things that Suzie may not be ready for, but may not know how to stop. What do you think? (and then really be open to her response)
The key is to just always always always have a dialogue going. I don't think this is a one-time, sit-down -on-the-bed between your husband and you and have an important conversation. She'll be dealing with this subject every day soon if she's not already. Make the conversation a constant, also.