I'm going to Japan in Dec. and I was wondering what kind of clothes I have to buy?

Our homestay student from last Christmas was, well, miserably cold. Just not used to the weather. And last year was exceedingly cold.

The key to staying warm is to layer. T-shirt, turtle neck, followed by a sweater and then a light jacket or even a coat. Have some light gloves to keep your hands warm. Shoes, well, not a lot you can do there. Open toed sandles would be a bad idea though.

It snows throughout most of Japan, so unless you're going to Okinawa or some place in the far south that's what you should expect. So bring boots and a good jacket. I'd also recommend long sleeve undershirts and possibly long underwear (which if you have none can be purchased in Japan at places like UniQlo for usually around 2000 yen). Waterproof pants can be nice but aren't necessary, I often just walk around in jeans and it's fine. Another most, though, are gloves or mittens, and a nice warm hat or beanie is preferable. Again if you don't have those you can purchase them here. 100 yen stores sell both gloves and beanies and they're warm enough and will likely last long enough for vacation purposes, and they're super cheap. But yea, boots and a jacket are very important.

It depends on where in Japan you visit.

I think you better wear a coat or something over a turtle neck.

Buy anime cosplay because most Japaneses dress like anime character.