my half sister

I am looking for my mother child she gave up for adoption in oct 3/4 1974. her birthmother last name is Thomas. I dont know where to begin to help find her. I dont even know if she tried to find our mother or even wants too. so this is a start I guess If anyone could help me. Birth mother is Susan Thomas Birth Father is Ken and I dont know his last name She does have 3 half sisters with her birth mother .