its simple really, you need to avoid the bad nutrients and take in more good nutrients, stop eating things like chips/cake and those "treats", its tough but it has to be done
eat more eggs, more milk, oats, etc.
most important thing is to avoid saturated/bad fats at all costs and take in a shit ton of protein, carbs (don't overdue it or eat the wrong ones), good fats, creatine, etc.
don't take my word for it, do your own research, and if you are serious, find a nutritionist to give you a meal plan
also run 2 miles a day or something, cardio is important to fat loss