I am a Cambodian citizen living in Siem Reap.

I am planing to go to Hong Kong for my birthday celebrating and visiting my girlfriend there. I have booked the ticket for round trip already So I would like to know how to apply the visa to get there? Should I go to travel agency or going directly to Chinese embassy in Phnom Penh? Is possible to apply online visa to Hong Kong for Cambodian citizen?

I am appreciated for any of helpful information that you can help.
Best regards,
Sina Yean