My niece and nephew were just placed in foster homes. How do I get custody? I want to adopt them. What is the first step?

The first step is to contact your local family services agency (the agency that placed them) and tell them you are willing to foster them. Once you have guardianship, you can then contact a Family Law attorney to process the adoption.

Was the agency that placed them aware that you were willing to accept them? Generally such agencies would prefer to place children with qualified relatives.