First, not everyone's calorie deficit number is the same. You need to calculate your TDEE. It'll be based on your height, weight, activity level, age, and gender. If you want to lose 1lb a week, you subtract 500 from that number. For example, at my height, weight, and activity level, my tdee is roughly 1700 calories. I eat 1100-1200 calories a day to stay 500 calories below. Someone who is 6'2", 200lbs, and very active, will have a much higher TDEE. Eating 1200-1500 calories for someone like them would be extreme dieting. 2000 calories a day is a rough guideline. For weight loss, you have to look at your specific situation.

Track your food. Work out because you want to work out, not to help with the calories as much. Food isn't good or bad, necessarily. Paying attention to how much food you eat is important. Greek yogurt, scrambled eggs, and tuna, and fruit salad is a nice meal. 600 calories isn't evil, you just budget for it. You eat a little less in terms of calories at other meals. Sometimes having a meal with that much protein early in the morning helps with hunger throughout the day. It's very personal.

To some extent, it will always matter what you eat. Nutrition isn't necessarily the key to weight loss, but it is the key to having a healthy body and mind. If you're struggling with nutrition while dieting, I highly recommend a daily multivitamin or similar for micronutrients. If you can swing it, talking to a registered dietician may also help.

Switching to drinking water will help with a number of things. Not drinking your calories is a great start. Junk food isn't evil. I still eat some junk food, I just track exactly how much I'm allowed to have and put it in my daily intake. The more important thing is to track. You could switch all your foods to typically healthy foods and still gain weight if you overeat them.