transit for 13 hours but want to see the Great Wall

I'm cambodian and i will flight to USA in this month, during the flight i need to transit at Beijing and wait there for 13hrs more. While for the waiting time, i would like to to see the Great Wall.

My question is do i need to apply for a transit tours visa first before i go there or i can apply transit tourist visa while i arrive to the airport? And how long for the processing visa?