Is it this Xmas and New Year you are looking at if so you'll need to get your skates on as flights get difficult to come by at this time of year. Its also the most expensive time to go. Accommodation in Phuket pre-tsunami used to get hard to come by at this time of year, I don't know if that is still the case.

So are you saying you want hotels rather than budget style accommodation? Would really need to have a feel for the kind of budget you want to spend to advise, whether you want to do it DIY or via a tour operator. Have a browse through some of the thread on this forum for ideas.

Plenty of things to do in Bangkok, pick a property near the Skytrain, Underground or river for ease of getting around, or maybe something near the Kho San road area where a lot of younger people hang out.

In Phuket the main resorts of Patong, Karon and Kata are lively enough - Patong particularly so. There are plenty of things to do on the island. Koh Samui..particularly Chaweng has a lively scene suited to younger folks like yourself. Weatherwise though I might pick the Phuket/Krabi area for beaching at that time of year.

Xmas/New Year classed as high season. Some hotels will charge extra for dinner etc.

Depends on your budget but some of the best hotels in Bangkok are on the river ie Oriental, Shangri La, Peninsular, Marriot. For Patpong Road look at Montien.

Phuket - Patong has the best nightlife so Holiday Inn, Amari, Avantika or just over the hill Le Meridian, private beach.

Lots of hotel reviews on Tripadvisor.

we have hotel reviews here

my first trip to Thailand was when i was about your age and I stayed on Kao San Road, popular with back packers. Hotels are cheap and cheerful, and can be used as a good base to explore the rest of BKK, although as other posts say, it does really depend on your budget.

I loved Kaosan Road area of Bangkok too, this is more Thai to me than other areas of Bangkok which are wall to wall skyscapers.
There is a great buzz in this area and the surrounding area . I like the look of Sawasdee House which is a small contemorary thai style guesthouse just about 2 mins walk from Khaosan Rd, there is a fantastic restaurant on the ground floor, it's lovely and very thai with great food.
May stay there next time we are in Bangkok , rooms cost from 500 baht per night.