There are three choices for you.
1.Take flights to Guangzhou at Hong Kong International Airport. It may take you 45 minutes.
2.Take Guangzhou-Kowloon Through Train to Guangzhou East Railway Station. It may take you one and an half hour.
3.Take Guangzhou-Hong Kong Express Coach at Hong Kong International Airport or other stations in the city. It may take you three and an half hours.
More Tips:
+ International Airlines
+ Take Metro to Canton Fair
+ How to get to Canton Fair when you arriving in Guangzhou