I think you should learn it in Hong Kong first.
You can go to Japanese school for 3 months.
If you really like it, then you just keep gong on.
And go to get Grade 4 Japanese Proficiency test; you can prepare for studying in Japan.
Moreover, you are not sure will you used to the environment; you can join some course that study in Japan for summer.
Those courses are short and can increase your Japanese level quick.
I think this web site is pity useful
http://www.yamasa.org/acjs/chinese/programs/index.html
I don't know which Japanese school in Hong Kong is better; you can try to ask people in here.
Some for thing remind you.
JPT only offer once a year in December.