Good luck with your chat! Good for you for doing this! It is a very important gift to your child.
In addition to figuring out what and how much to say in your first talk, you may wish to consider getting a book for your daughter. This way, she could look at it if she has any questions but is shy about asking. Also it gives you something to look at together, and reminds you of the key topics. You can find many books on the topic, and one is likely to suit your values and her reading level. I like the book "It's So Amazing! A Book about Eggs, Sperm, Birth, Babies and Families" by Robie H. Harris. I find it has all the basic facts, but is cheery and has nice cartoons.
Don't feel like you have to cover everything in one talk. I think a shorter talk is good for a first one, as long as you get back to the topic some other time.
With a smaller child, some parents I know described sex in their first chat as "a special kind of close hug that feels good." But a ten year old might be too old for that kind of euphemistic description! (I added that we do not hug everyone!) With an older child like yours, I would share some basic values about intimacy, and a short comment about protecting your health (the same way we don't kiss people who have colds.) But I save the details about condoms, birth control, STD's and risk for a later age, except to say there are ways of preventing having babies, but we will save the details for later.
I also saved the "emotional side-effects" part of the chat for a later age with my older son, now 16. I kept to the simple facts with both of my sons in the first few talks. I try to have at least annual updates with them, or to make sure my husband does.
I find it helps to admit it can be awkward or embarrassing to talk about some of the details, if you or your child seem to feel awkward or embarrassed. And needless to say, talking about sex with boys can be awkward for a mom or son. That being said, I think it is a very important thing for moms to communicate with their boys about this. I can't speak for dads with their girls, but I think since most kids turn out to be heterosexual, learning that it is OK to speak about this topic is a big investment in your child's future ability to communicate with a potential spouse or life partner.
I also tell them that this is a topic that is private, and not to be discussed with children their age, that it is not fair to friends to talk about this topic, because it is their parents' job to do that. Once they are 14 or so, they do talk about it with each other. But at 8 and 10 and 12, I think it is important to tell them that people have many different values and opinions about this topic, and parents should be the ones to talk to their children about it.
I hope any of this is useful to you. Good luck and best wishes!