To be honest you will be doing more harm than good with a fast. There is no such thing as body detox diets or those other similar worded fasts and such. It is snake oile salesmen looking for a quick buck. Your liver, kidneys, and skin do the work of ridding your body of harmful things. Your body needs sustenance to properly function. Without proper diet of at least some food your body will enter a survival mode in which it will start to consume itself for energy. That is not good as you will then have to deal with the out of whack body systems. Simply do not fast.
Instead avoid the foods that cause you the problem. Of course that is easier said than done. But if there is no junk food in the house you cannot go straight to it. You will need to start incorporating the more healthy foods into your diet. Try vegetables and dip rather than a bag of chips. Order the side salad instead of the plate of fries. There is no need to overhaul to rabbit food and overwhelm yourself. Just go slow and it will become easier.