Making a body bigger or smaller is 80-90% diet. My exercise routine doesn't change whether I'm building muscle and fat or retaining muscle and losing fat, what does change is my diet. Exercise helps, for sure, but your focus needs to be on what you are eating.
I highly recommend everyone try and learn about nutrition to some extent. I don't expect people to have my sort of knowledge but people should know at least something about a task as fundamental as what we eat. Just read up on things like BMR (Basal metabolic Rate), TDEE (Total Daily Energy Expenditure), and basic macronutrients. Learn the difference between saturated and unsaturated fats and the difference between sugars from fruits and vegetable and refined high fructose corn syrup in just about everything else. Gaining a basic understanding of how nutrition works is so important.
It will all boil down to eating clean and eating in moderation. My philosophy is there are no healthy or unhealthy foods, only healthy and unhealthy amounts of food, i.e. apples are good but nothing but apples is bad, a burger now and then is reasonable but nothing but burgers is bad.
Another rule of thumb is to buy foods that don't have barcodes. Meat, fruit, and vegetables should be making up the majority of your diet. Things that come in jars, boxes, tins, etc. are things to keep to a minimum.
The real key with weight loss is to not have it as a diet. Diets are inherently temporary and their end will cause instant weight gain once returning to old patterns. What you want is lifestyle change. You want to get yourself into healthy patterns that you enjoy and that become a part of your life permanently. By doing this it means you won't have to worry about tracking things, the weight loss and maintenance will happen autonomously.
So diet is the key but exercise is highly recommended. The thing to keep in mind about exercise is it is not a case of more is better. Lifting every day and/or running for hours on end is counter-intuitive; it leads to huge muscle catabolism (read: skinny fat) and increased risk of injury. The saying is "gains are made on the rest days, not on the workout days" so be sure not to push too hard, take rest days, sleep well, and be patient. By doing things right you ensure yourself of long term, permanent results.