How to get the Subway from Railway Station in Beijing?

I nead to take the subway from Railway station in Beijing and go to the Beijing Capital Airport.
Can You give me more details about this, regarding the distance between the railway station and subway, from where i get my ticket, how many bus stops I have till I arive to Beijing Capital Airport.
I'm traveling from Changsha (Hunan Province) and I nea to arive to airport to take the plane.
Thank you in advance