I am a nepali citizen having student visa of japan.i want to visit HK for 1 week .how can i apply

Firstly, prepare the required documents for tourist entry permit: copy of your passport, a recently-taken passport-style photo, a completed application form, proof of your financial standing, hotel reservation, flight itinerary and your travel plan.
Secondly, mail your application to HK immigration department to apply for it. Here is the postal address:
Receipt and Despatch Sub-unit
HK Immigration Department
2/F, Immigration Tower
7 Gloucester Road
Wan Chai, HK