Cantonese and Mandarin are both called dialects for political reasons by Chinese governments, but in reality the linguistic difference between the 2 are similar to that of English and Dutch, or French and Spanish. Not as close as Spanish and Italian. The 2 are not mutually intelligeable although its easier for speakers of one to learn the other, just as its relatively easier for a speaker of french to learn Spanish versus a Japanese speaker learning Spanish.
Life will be challenging and fun, but not impossible. A few years back a European student wrote up his life after a scheme exactly like yours where he'd ended up becoming fast friends with his host family and learning basic Cantonese. This will be a once in a lifetime immersion experience, and highly worthwhile in the friendships you will make and what you learn about yourself.
Wouldn't the organisation sending you here have previous 'alum' they can put you in touch with?
Things to expect - heat, humidity and crowded living conditions. Along the lines of a bunk bed in a small bedroom shared with at least one other person. Observe basic rules of considerate behaviour - look at how others behave at table, around the house, etc and imitiate. You will also have relatively little privacy compared with the West - family space is public and shared, including teenagers' bedrooms. Your host family should be friendly and helpful - why else would they invite a stranger into their homes in this crowded city? See if there's anyone in the family you can email ahead of time to build a rapport with.
School rules - uniforms are worn, and most schools/students take pride in their uniforms. Observe those rules and you'll avoid antagonising the staff. Short hair for males - usually there's something about hair not touchign the collar. No jewllery apart from wristwatches. If your school is English medium, most of the classes will be conducted in English. If it's Chinese medium - more of a problem as even the text books will be in Chinese. Do your best to ensure you're allocated to an English medium school. In general, HK secondary school standards are competitive with worldwide standards, particularly in the sciences, so I wouldn't worry about falling behind if you're assigned to the science stream in an English medium school.
Basic classroom manners to avoid antagonising staff and fellow students - raise your hand before addressing any comment to the teacher and wait for permssion to speak. Check if the other students stand up when asked to speak and if they do, you should observe the same courtesy. Again, show sensitivity about how many comments and questions you pose in class - unfortunately the syllabuses are dense, class schedules tight and class sizes large, so it may become counterproductive if your questions and comments dominate the whole time. Usually I'd say 2-3 questions max on a topic if you're lost, and then follow up after class with the teacher for further questions. Of course, if the teacher asks if there are any questions and you have some, go right ahead as long as you don't take up the whole 30 mins for class. As for most boys progress at sports is valued, and if you're good at soccer or basketball or table tennis those are all great icebreakers. Schools don't worship sportsmen here to the same extent as in the US, but male students usually appreciate competence in sporting activities. Most schools have a wide variety of after school extra curricular activities, usually run by the students themselves - get involved in these and it'll be much easier to make friends.
No, people won't go ga-ga over westerners, HK was a colony for 150 years and experienced a lot of negative behaviour from all types of westerners at the group and individual level, so show some sensitivity to that.
General social behaviour - smile a LOT, all the time, and greet people, even if they seem to ignore you at first. Smiling is important in Chinese socieity. Ask your host family for guidance as much as you can, they'll find it flattering. Heavy drinking among young people in high school is frowned upon as anti social and unheathty, so be careful about drinking even beer within your host family unless they offer specifically. And if you go home drunk they will be very seriously worried about both your wellbeing and your effect on their kids. Ditto drugs. of both hard and soft variety.
Bring books/CDs/videos/magazines in your native language for when the homesickness gets bad.
Cantonese is not impossible to learn, most people don't try very hard because they believe it's either impossible or not necessary. A young Italian guy I know who had zero English was fluent enough to compose and deliever a 20 minute speeach in Cantonese after a 1 year course at Chinese University HK. So you'll be able to pick up at least a smattering if not more. The more Cantonese you try to speak, no matter how bad you feel it is, the more positively everyone will respond to you.
Best of luck, this sounds like a wonderful adventure.