Don't worry too much; most people put on some weight over a holiday or a festive season, and once you get back into your normal routine it should start to get back to normal anyway.
But the extra pounds can linger for some weeks or even months, so there a few things you can do to speed up the process:

Get straight back to exercise. No matter how cold it is or how tired you feel, do something every day - even just walking round the block or doing a few stretches will get you started, and then you can do more.
Give away all those boxes of cake and sweet stuff which are still hanging around the house. Take them to work, give them to a community centre or church, but don't keep them.
If you don't cook for yourself, start; ready meals are often very fattening, if you do cook, find new and tasty ways of cooking with more fruit and veg. Eating 5 portions of these every day is much more effective than dieting.Hope this helps.