D.J.
There are some wonderful books that you can share with your child about sex. You can add one during your regular reading time routine. Don't get the ones that are so detailed, that's for a bit later. Also, I found that just answering questions as they come up helps. By seeing animals mate or seeing baby animals being born, this creates a interest and natural curosity about life. Most important relax and let it a natural part of life.
Happy Parenting!
D.