Figuring out how to talk to your child is even more important than discussing adoption with your friends and family. It's important to start early and make the conversation age-appropriate; don't overburden your child with information he or she can't understand, but don't keep secrets, either. If you adopt a newborn or infant, begin using the word "adoption" from day one. As your child observes pregnant women and learns about babies, he or she may ask you about his or her own birth. Then, you can say that he or she grew in another woman's "tummy" (or "uterus," or however you'd like to word it) and explain that she couldn't take care of her baby, but wanted him or her to have a loving family.
When your child begins school, you may want to tell his teacher that your child is adopted so that discussions of different types of families will be inclusive, but you don't need to go in depth. How much your child wants to share with classmates is up to him or her. Many adoptive parents use books to help their child learn about adoption.
The older your child gets, the more he or she will understand adoption, and more questions will arise. Emphasize that you are his or her forever family, but avoid disparaging the birth parents; they'll always be a part of your child, and he or she may internalize your negative comments. Arguments during the tumultuous teen years may include comments that you're not your child's "real parents." This may be hurtful to you, but realize that he or she is also working through emotions about being adopted.