1) Relax. They will pick up on your anxiety, especially if they are older than 7 or so. YOU are the one bringing a lifetime of experience to the conversation and this can make you tense, afraid, and anxious because you know how sex can be both great and awful. Calm down and step up.
2) Reflect on your own experience learning about sex. Do you want the same for your child? My guess is probably not. What are your hopes, dreams and goals for them as they become sexually aware and (eventually) sexually active? Tell them.
3) Tell them sex is a natural, normal and healthy part of life. And you’ll be talking about it, whether they want to hear it or not. Teach them what you should have learned about sex as a young person, not whatever it was you actually did learn.
4) Acknowledge their discomfort with the topic and your own. And then keep on talking. They will live. You will too. No one ever died because their parent had consistent conversations with them about sex, love and relationships.