Both. Weight management is a simple formula: calories in vs. Calories out. A sensible diet combined with regular aerobic exercise will keep most people at their ideal body weight. All else being equal, it takes a lot of exercise to reverse a bad diet: one donut can take an hour of jogging to burn off. If your goal is weight loss, this is best started with professional help: ask your doctor for advice.
Both needed. There are three components to weight management. 1. Proper nutrition - ideally based on predominantly whole fresh food and less process food, animal protein or dairy. 2. Daily exercise comprised of cardiovascular activity and strength training. 3. Detoxification and alkalinization of the body - rid body of harmful toxin and acidic ph. Also important drink close to 100 oz water daily to flush.
Both. One doesn't work very well without the other. Nutrition is with you your entire life. There are many, many resources available to you. There are books, classes, webinars, lectures, etc. You can hire a nutritionist to help you. Likewise, join a gym and go there 5 times a week. Hire a trainer for the first 2 or3 sessions to show you what to do. There is a lot of help out there for you.