I would like to visit to Hong Kong as a tourist .how can I ask the visa for Hong Kong with Cambodian

Linna, as long as you provide all the required documents, I don’t think it will be that difficult to apply the same next year. Just have a try.
First, prepare the required materials including the filled-out application form affixed with a passport-sized color photo taken recently, copy of your passport, proof of your financial standing, transportation arrangement, and your travel itinerary to HK. Next, submit your application to the embassy of PRC in Cambodia to apply for it.
As for the fee, you need to find it out by yourself. As far as I know, if you post your application to HK immigration department directly, it charges HKD190.