How do I legally adopt my step daughter what steps do I need to take I'm caring for her

You would need consent from the biological mother or father, they have to sign over all parental rights to the child, and then you would be responsible for them... child support (if you are the step-father and get divorced) etc.

You file a motion in Family Court to adopt her. The biological father has to be given the opportunity to consent to the adoption. If his whereabouts are unknown, then every good faith attempt has to be made to try and contact him. If he cannot be found then the adoption can proceed without his consent.