Birth certificate

Well, to adopt the child legally, you will have to enter into an 'adoption deed' with the guardian of the child. Assuming from the question it's the grandmother who is the guardian currently, you will need her consent for taking in the child for adoption. Note, if you are married, you will also need the consent of your spouse before you can legally adopt the child. Also, as per Hindu laws, you must not have a son before you can adopt. Having a daughter, however, would not pose any problems with the adoption.

You can have an Aadhar ID made for the child after successfully completing the above steps.