You need to apply for it in advance through Chinese embassy in your home country. It takes 4 working days for visa processing. The requirements are:
1. Your Passport/travel document and passport photo.
2. One completed Hk application form affixed with one passport-sized photo.
3. Hotel reservation, return ticket.
4. an invitation letter in which the sponsor agrees to provide financial support and accommodation, and one copy of the sponsor's HK ID (if any).