How can you get your kids to talk about sex?

Its not like you can force them but I feel I can't guide them at all because they will not talk to me about sex at all. My oldest I suspect is active but I don't want to assume it but I want to be able to give advice without fighting. Also concerns about her safety as well. I have told her she can tell me anything and I have asked questions but I just get attitude answers. I know I couldn't talk to my parents about it but they didn't even give me the option really. Tweet