There are some medical exceptions but the cause of weight gain for most of us is that we love food too much. Don't blame your weight gain on your "slow metabolism!".......A slow or sluggish metabolism is not the major cause of weight gain.
A caloric imbalance is the cause of weight gain (consuming more calories than you burn in a day). If you want to speed up your metabolism and burn more calories then exercise regularly (strength training and cardio exercise). Also, eat right and often. Eating small meals every 3-4 hours will keep your metabolism humming and will keep your body from shifting to "starvation mode" (storing fat).
Do you need help with meal planning? Don't wait until January to do something about your eating habits! Start taking steps now! The last time I checked, we all eat every day!