Common consideration always works, as does common sense.
Help as much around the house as you can. Watch what the other family members are doing (especially the mother) and help them with those tasks. They'll politely tell you not to bother, but keep insisting and tell them you're used to this at home. Your offers of help willl be well taken and after a couple of weeks they'll stop being polite and let you help. Clean and tidy up after yourself, it's great you already help out around your house.
"Any special sport of hong kong or china that's populuar?" like I said, table tennis, soccer and basketball are the most popular. Squash & swimming is also common. Otherwise it depends on what access people have to sporting facilities. Get involved in school sporting clubs and activities to make friends.
Please note that the HK education system is very competitive so most students do spend anything from 2-5 hours a day on schoolwork, depending on ability, aptitude and which year they're in. How old are you and what form are they putting you in?
"and z75.. about that smiling... what should I do if they ignore me? should I speak to them even so, and smile?
If someone did that to me when I dissed them I would think it to be quite irritating:P"
Apply common sense. I'm not telling you to force people to have conversations with you, just smile at people and greet them when you run into them. They may be too surprised to respond the first few times but eventually they'll think of you as that polite young man and start smilng back. No one will diss you for just smiling at them.
" Do youths in HK have sex? Where in that case? Since homes are a bit crowded."
In general not seen as the appropriate thing to do for high school students as they're not yet at the level of maturity to be able to deal well with such encounters. Again, you'll probably worry your host family a lot if you engage in these activities as they'll be concerned for your wellbeing.
BTW please note that personal space is treated differently here than in the West - it's ok to stand close to or touch your same sex friends but do not touch female friends casually or people will think you're taking advantage of them. You can hug the guys but not the girls. BTW most schooling in HK are through single sex schools - are you going to a co-ed or single sex school?
"Is it a bad behaivior to go to a party, if not, is it a bad behaiviour to drink just a little, social drinking, not get drunk."
As another poster said, it depends on the family. Also on the type of party. Ask your host family for their preferences and guidance. Again, generally not a lot of alchohol consumed by high school students. I really would avoid even social drinking unless your host family is specifically inviting you to do so. Alchohol consumption by high school students is usually seen as something unhealthy at such a 'young' age, so you run the risk of worrying them as they will feel responsible for your well being. Light social drinking by university students or working people is fine.
Why don't you get some Cantonese language tapes and see how much you can pick up ahead of time?
Also, see if you can contact the school and ask if any of the students in your new class would be interested in emailing before you arrive. The English teacher might be happy to help with such arrangements.