Kathmandu, Nepal to Beijing (layover)

Hi,

I'm a Canadian citizen and will be travelling from Kathmandu, Nepal, > Guangzhou, China (connecting flight) > Beijing, China (layover for 18 hours). Then will be back to Canada next day: Beijing > Guangzhou > Vancouver.

Question: Do I need a visa and where should I apply (Guangzhou or Beijing). I'm planning to do the layover tour in Beijing but will be coming from the domestic terminal because of my connecting flight from Guangzhou.

Thank you.