yes, you need a visa to Hong Kong if you are Pakistan passport holders. You can mail your application to:
Immigration Division of the Beijing Office of the Government of the HKSAR
71, Di'anmen Xidajie, Xicheng District, Beijing 100009
Telephone Enquiries (852) 2824 6111

For more information, please refer to http://www.bjo.gov.hk/eng/pgm_index_e.htm

thanks. one more thing which type of visa can i apply and what documents required.

You can apply for Hong Kong Visit Visa which is issued to aliens who come to Hong Kong for sightseeing, short-term business and other private purposes.

The requirements are:
Application Form for Entry for Visit/Transit in Hong Kong (ID 1003A)

your recent photograph (affixed on page 1 of the application form ID 1003A)

your passport and its copy containing personal particulars, date of issue, date of expiry and/or details of any re-entry visa held (if applicable)

Photocopy of proof of your financial standing, e.g. bank statements, savings accounts passbooks, tax receipts, etc. and proof of employment (if any), e.g. company leave letter, salary slips, etc.

Photocopy of proof of transportation arrangement, e.g. flight itinerary

Photocopy of proof of the proposed leisure visit to Hong Kong, e.g. receipt of joining a package tour to Hong Kong, itinerary, etc. (for leisure visit only)