Most parents are afraid to talk about sex with their children, thinking it might increase their inquisitiveness but it's best "they are ready" if not the world will tell them about it.
So firstly when they are baby (1 & 2yrs) start by teaching them body parts (naming them, showing them their nose, mouth, Private part etc. Both boy's and girls have different private part and No one else is allowed to touch their private part just let them know it.
3-5 yrs:- Teach them to say"NO" to hugging, kissing cause they are incharge of their body, they should be able to differentiate between good and bad touch and they should respect elders but any elders touches them anyhow they should scream and report to their parents immediately.
6-9 yrs:- this is the best time to talk about sex because they are exposed to the internet at this age and peers too.
10-12yrs:-this is the time they go through alot of physical change in their body like menstruation, growing hair,wet dreams etc
Lastly create a friendly environment where the child is free to share anything with you and be their best friend.