...December 14th I have to be off for 1 week. What should I do? Will I get fired since already I need to take a week off?
You should have told them about the week off before you took the job, in your interview. If they had employed you knowing about that before you started it would have been ok.
I suggest you read your employment contract if you even are allowed a week holiday already. You might need to ask for a week unpaid leave, but yes they will not like that and, depending on the employment laws where you live (check what your rights are), it might lose you the job. But just talk to your manager about it asap and explain. Good luck.