I'm a US Citizen. I'll be traveling from Newark, NJ with a 2 hour layover in Beijing

then on to Guangzhou. While I'm in Guangzhou, I'll be taking the express train to Hong Kong. I'll only be in China a total of 6 days, leaving out the night of the 6th day. I can't figure out if I need to purchase a Visa, or if I don't. I'm very confused, I've gotten conflicting information from several sights. Can someone help me, please? Also IF i have to purchase a visa- what source should I use?

Lexi