I want to apply for HK visa via China embassy .

Reza, if you go to HK for sightseeing purpose, you should first prepare those documents including copy of passport, financial support proof, a completed application form attached with a passport-style photo, travel itinerary, and proof of transportation and accommodation arrangement.
Next, submit the application to China embassy. If your application is processed by the embassy, it takes 4 working days to get the result. However, 4 to 6 weeks may be needed if the application is transferred to HK immigration department.