I adopted my child when she was 3 months old and she will turn 2 next month. When do you think I should tell my daughter that she is adopted and how should I do it?

You could get some books which the both of you can read together. She may have a lot of questions about why she was adopted and if her biological parents didn't want her anymore. Reassure her that you will always love her no matter what and she will understand that you don't need to share the same blood to be family. She might want to know about her birth parents when she grows older, but that's her choice to make. http://www.parents.com/parenting/adoption/facts/childrens-books-about-adoption/