How do you all start your diets for the weight you gained suddenly after the holidays?

I ate to my heart's content during the holidays, and my weight increased in no time. I feel like if I let this continue, it will turn into actual fat, so I’ve made up my mind to start a "quick weight loss" diet today. How do you all begin managing your diet or exercising right after the holidays? Do you cut down on your food intake immediately, or do you start by increasing your cardio? Please share any tips or know-how you have!