Do I need to apply for visa to Hong Kong? Or, Am i going to have visa on arrival?

Ahmed, you don't need it.

Thanks for your reply, If you excuse me I have two more questions:
1- How much cash (approximately) I should keep with me?
2- Do I need to keep other documents beside my passport and the return flight ticket?

To satisfy the immigration officer on arrival and I meet with the normal immigration requirements and qualify for entry as bona fide visitor.
Regards,

Ahmed, you need to prepare USD 100 per day if you want to visit HK.
You also need to provide proof of financial support beside passport and the return ticket. Sometimes, the hotel reservation may also be needed.

if i have a reservation for an appartment that's fine? or should i book in hotel??

That is fine. You don’t need to book a hotel anymore.

thanks so much for your time and helpful answers Cindy :) :)