Well first of all Hong Kong dialect is Cantonese not Mandarin some people in Hong Kong can understand Mandarin but not all of them. but It doesn't matter really. a lot of Hong Kong people can understand bits and pieces of English already and If that don't work use sign language :) that's what I always do especially in the Cafe de Coral fast food chain and If your planing to go to the flea market or night market in Hong Kong and if the vendor can't understand you he/she will type the price in the calculator and then show it to you. you can also type in the desired price if you want to haggle. but beware some vendors are rude. so be sure you want to buy the item before you start asking questions lol. I had an experience a couple of years ago that a vendor cursed the heck out of me when I asked him a couple of question and decided not to buy the item because it was to expensive. by the way the rudeness only applies to the street vendors/night market vendors. If your gonna go to a mall or boutique people are nice there.
Heck I've been to Hong Kong many times already and I never had any problem. Just get a map from the airport when you arrived its FREE. make sure you take a couple of them just in case you lost one. Also take some Hong Kong Visitors Kit/Guide booklet in the airport and they are FREE also. MAKE SURE EVERYTHING IS ENGLISH coz they have other version like Japanese and Chinese. =)
Navigating Hong Kong is easy. If you love walking just be sure to always take Nathan Road. I think that's the main road and when you get lost just ask where Nathan Road is. Also you can take the subway. I think they have Subway Maps also in the airport but I can't really remember. If your planning to take the subway thats where the Hong Kong Travel Kit comes in. It shows you where and what to take and where to exit. =) Just don't forget to read the signs. I had that mistake a couple of years ago and ended up taking the long cut instead of the short cut. =)
I'm sure you are already aware that Hong Kong has 2 Island. 1 is Kowloon side and the other one is Honk Kong Island. I suggest you go book a hotel on Kowloon side rather than Hong Kong side. coz for me Kowloon is where everything's at Night Market and stuff. Night Life and etc.
You can check out this map
http://www.centamap.com/gc/home.aspx to plan ahead =)
Oh yeah I'll be visiting Hong Kong also this coming Dec 19 to Dec 24 =) and by the way Good Luck and Congratulation on your graduation =)