The Guangdong area's farmland doesn't really support growing tea. It's too wet and hot here. What they do specialize in is herbal, medicinal tea.
You'll see a lot of tiny shops selling tea in containers like this, or they'll have 5-6 big metal teapots brewing. You can ask your hotel to recommend you one and write it down in Chinese, or just use the point-and-buy method.
Even easier, Wong Chun Loong Herbal Tea can be found all over China but originates here. The teas are sold cold, in plastic bottles. Here is one location, but you'll find them all over the city.