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Unfortunately for you, "just finished school" is a typical situation for young people to leave home to seek their fortune abroad -- especially if home is a country with low socioeconomic development. The main task of visa officers is to keep those people out -- so in order to get a visa you'll need to present a convincing reason why seeking your fortune in Hong Kong can't be your plan.
Of course you'll need to show that you can afford a pleasure trip to HK, and you would need your sponsor's information for that. But in the big picture that's a relatively minor hurdle.
You'll need to show that you have a promising future to return to in Cambodia. If you don't have a job lined up, being accepted for a university or other further studies outside HK may do, if you can also explain how you'll pay for that education -- i.e. your living costs and if applicable tution fees or the like.
But if you don't have that, you chance of qualifying for a visa is -- by design! -- very slim.