Exercise is like a billion times harder than just cutting down on calories. Only changing your diet will change things (100% truth).
Do one week where you literally weigh everything you eat and drink to calculate your calories for each day. 1 week of hard calculations on your calculator and possibly an excel sheet on your computer. It's actually extremely easy once you get into the flow. But one needs to start (the hard way).
I've never been "fat" but I once calculated my daily calorie intake to be 4000... that's like 2000 too much.
Cut 1000 calories each day and you will lose 1 kg a week (guaranteed).
You could of course cut 500 and then exercise and use another 500. All the same (but exercise is probably way too hard).