I suggest you make your own itinerary rather than go to an agency. With a tour package, you will most like be taken to a compulsory city tour which includes a trip to some kind of jewel or souvenir factory where you will be forced or convinced to buy. This is just a waste of time.
When I went to HK last September, I made my own 5 day itinerary which included a trip to Macau. I visited more or less 90% of my planned locations and most of my planned activities.
Best place for first timers and even most tourist is TST area (Tsim Sha Tsui). This is where most of the action is. The closer the hotel/hostel of your choice to the MRT station, the better.
MRT will be your main mode of transportation in >90% of the time (you will need to ride a bus to go to Ocean Park but aside from that, it's MRT all the way even to disneyland).
Suggested 3 day itinerary
Day 1 - Ngong ping or Victoria peak (If you travel by CebuPac and arrive midnight or early morning, you can go to Ngong ping village (via cable car) during the morning and visit Victoria peak (via tram) during the afternoon/evening
Day 2 - Disneyland or Ocean Park; If you like shows and a disney fan, it's a no brainer and pick disneyland. However, if you're a thrillseeker and likes animals, you got to go to OP. But if ask me, if I had to pick one, Disneyland wins.
Day 3 - If you like shopping, I suggest you dedicate your last day for it. There's a lot of shopping options in HK, ranging from tiange2x to Outlet shops of mid-range brands to expensive brands.