do i need to apply for a visa for hong kong and for mainland china?

HI i am a Usa citizen. My mother and i am planning to go to hong kong to visit family. i will be staying with them in their apartment. i bought my plane tickets to stay in hong kong for 3 weeks . It is my first time going to china, Also i read that you need to have plenty of funds to stay there, how much do we need in funds to stay there for me and my mother? Thank you