How to begin?
Don't Feel Awkward- What your child learns from the conversation will be decided by how you deal with it. Don't fret or appear nervous, the more confident you behave the better will be the reception by your child.
Don't Wait For The Perfect Opportunity- There is no need to setup a meeting. This is not a business meeting! Take casual opportunities as golden chances. Be direct from the beginning, the time for metaphors are over. Remember to always consider your child's point of view too.
Be Sensitive- Remember it's not just information you're transmitting. Don't just talk, try to make your child understand the situation and the reasons. Use a suitable mixture of logic and emotions.
Which topics to hit?
Sex is a natural phenomenon. Make them understand there is no good or bad in it. It is not just for pleasure but also has several positive effects on one's health. But casual sex without any protection could be very dangerous.
Masturbation is safe.It is the simplest way to achieve sexual pleasure with the minimal risks involved. It will make them more familiar with their body. But it must not interfere with their academics or work. Porn is also an important topic. Don't force them to stop watching it altogether (this will only encourage them). Help them improve their outlook towards sexual relationships.
Puberty is confusing.You've been through it so there is no need to explain the whirlwind of emotions and hormones our body goes through during this period. Their body will be changing and you must make them believe that it's normal. Stay with them during their transitional phase.
Peer Pressure can make them choose poorly.Many times, children have chosen the wrong choice knowingly just to fit in. Tell your kids that it is not okay to be submitted into having sexual intercourse. It has to be their decision and not a matter of social status.
Contraceptives are important. It is the best available option to avoid pregnancy. From condoms to pills, talk to them about its benefits. Tell them about the threat of sexually transmitted diseases and how they can be prevented.
Related - World Population Day 2017: Top 5 Myths About Birth Control