I think this winter is warm in Japan

I think this year's winter is a warm one for in Japan

I think this winter is warm in Japan. GOOD JOB!!!

It's winter now. GOOD JOB!!!

I think It's warm this year in Japan because I don't use oil heater in this winter's nights. ---this sentence is wrong. what you are saying is that Japan is warm because you are not using oil heater.  

I think you meant this: This year, I think it's warm in Japan, because I didn't need to use oil heater during the winter nights. 

When it was about this time last year, I had already used the heater. ---This sentence is grammatically correct!!!!

You can say it this way as well:  During this time last year, I had already used my heater.

By the way, I think that it is the best as the bedroom of the heater but it cost a lot in electricity. --- You are saying that the heater had a bedroom and the bedroom costs a lot in electricity.  

I think you meant to say this: By the way, this is a good thing because heating the bedroom (wastes a lot of electricity) OR (costs a lot money). 

Don't get discouraged! Keep on working hard!!!!