How do I begin to talk with my children about sex?

I have three young daughters (ages 8, 6, and 3 years), and I know that it is about time to begin teaching them and talking to them about sexual issues. But my mother never talked to me about such things, so I really don't know where to begin. I do need to do it soon, though, because I was molested as a child and I don't want my girls to go through the same thing, you know, thinking they have no one to talk to about such things. Because my Mother never believed me when I told her that someone had molested me, I want my girls to be able to talk to me about anything. But I am very shy and easily embarrassed by certain issues, such as sex. How can I overcome this, and how do I spark a conversation about sex with a six-yearold, what should or shouldn't she know, and how can I explain things so she will understand? Same thing for the three-year-old.