I am a cambodian passport holder , and i wanna visit china and hong kong.

Hello, dear, you need to apply for two separate entry permits to mainland PRC and HK. The documents for the two entry permits are almost the same, so you need to prepare two sets of applications.
If you go there for tourism purpose, the required documents include your passport and copy of the data page, the application form attached with a passport-style color photo, round trip air tickets and hotel reservations as well as financial support proof and your travel itinerary.
If it’s your first time to apply for PRC entry permit, you also need to provide the household registration certificate.