Transit passenger wants to visit HK?

I am a transit passenger, with US passport, do i need visa to go out? Also, I would like to visit my cousin in Kowloon station. How do I get to Airport express from Airport? Where do I buy the ticket? Before I buy the ticket do I have to change currency, if so how mush US dollar should I exchange?
Last I will be coming back from Kowloon to Airport, anything special I need to know?