So many people feel the same at the moment! January is the worst month of the year for a lot of people in the northern hemisphere.

To start with; it sounds obvious, but you must make sure you aren't ill. There are so many viruses around at this time of year. If nothing is actually wrong, then it's probably a question of eating lots of fresh, light food to give you energy and undo the harmful effects of Christmas, and getting plenty of exercise. Especially, we often don't get enough daylight at this time of year and this can cause Vitamin D deficiency, so rather than going to the gym why not just put on your outdoor shoes and go for a quick walk? Even 15 minutes in the fresh air will make you feel much better. You can find more exercise tips here.
Also, if you start an activity just for fun - dancing, a new hobby, or similar - this will boost your energy levels too.