I am a Nepalese citizen girl &I want to apply for Hong Kong visa but how can do that?

Sabina, if you go there for sightseeing purpose, the first step is to prepare the relevant documents including copy of passport, the application form affixed with a passport-style photo, proof of your financial support, travel itinerary, flight tickets and accommodation arrangement. Next, post the application to HK immigration department. Below is the postal address:
Receipt and Despatch Sub-unit
HK Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, HK