The government of Hong Kong has adopted a bi-literate and trilingual policy, meaning that Chinese and English are now both considered official languages, with Cantonese as the official spoken variety of Chinese, while also accepting the use of Mandarin, often known as Putonghua.
Most Chinese speak Cantonese, but some other Chinese dialects such as Shanghainese and Chiu-Chow are also used. The majority of Hong Kong residents speak Cantonese as their native Chinese language and English as the official language. Nowadays with the Chinese control of Hong Kong, Mandarin is being spoken more than ever. Also, people are being encouraged to speak Mandarin due to the increasing communications and business with Mainland China.
English is taught in schools and widely used in the government, legal, professional and business sectors, as well as in tourism; taxi drivers and salespeople are able to communicate in English so you won’t have any problems if you don’t speak any Chinese!
As for the signage, all the official signs are bilingual (English and Traditional Chinese) and most shops and restaurants have English signs as well but do not expect it in rural areas.
Communication is not only a matter of language, but also cultural aspects, so regardless of the language you are speaking, you should keep in mind that Hong Kong Chinese uses a lot of non-verbal communication. For example if someone sucks air through his/her teeth while you are speaking, it means that they are unhappy with what you have just said!