Workout wise we would need to know your aims and potential limitations. Mobility issues, injuries, heart conditions etc. I know you are young enough to have avoided these but such rapid weight gain can cause some of these and I would rather be safe than kill you with advice you shouldn't follow.
Diet wise try grazing on complex carb based foods. A bit of junk interspersed as a treat will keep your body happy from cravings and dramatic diet shifts will leave you feeling very dissatisfied so accept that there will be junk in your life for a little while yet. Aim to cut it to weekly or monthly treats over the course of the next months or year.
Sugar cravings are caused by you hammering your insulin cycle and the steady intake of complex carb foods will stop this very dangerous process. I am not exaggerating the risk here, type 2 diabetes is the fastest growing disease in the world today and leading cause of blindness, something to take seriously.
All carbs, in fact all energy foods including fat and protein (yes protein has calorific value, equal to carbs in fact) are broken down to glucose. Sugars are one or two molecules of glucose linked together, so fast to break down and absorb into the blood, complex carbs aka starch is a long chain of glucose molecules that still gets broken down to glucose and absorbed into the blood but takes a lot longer to do so, the difference is massive.
A sudden rush of sugar in the blood stream from sweet food means the body goes into emergency mode and releases a mass of insulin to get it under control. By the time the issue is dealt with the body will still be releasing insulin so shortly after the sugar rush there will be a period of lull where the body has been stripped of more glucose than it had to start with. The bodies way for dealing with this is to crave more sugar to top up the levels in the blood, and of course answering this with more sugar starts the process over again. Failing to answer this means your body is low on available energy, and that makes you tired, lethargic, snappy and short tempered.
Starch releases the glucose over a period of time, so the body still needs to release insulin to control excess but the flow is slower and more predictable so it can tune this safely and easily. There will still be sweet cravings on occasion because of the diet you have been used to but these will fade fast as the body starts appreciating there is always a supply of glucose in the blood.
This is easier to achieve than you may expect. I sit at my desk every day with a rice steamer that looks like a bucket with rice and either eggs, meat or something else in there with it. Throughout the day I shovel in a forkful at a time. A few in workplaces have copied this with more interesting flavours, adding curry spices etc. or substituting rice with pasta, potato salad or sandwiches they took small bites of throughout the day. I tend to say I have 1 meal a day taking breaks for sleep and training, and this system has worked for me for a couple of decades now.
Additional bonus, it's cheap. Many diet plans are crazy money asking for the most expensive hard to come by ingredients, this isn't. You adjust the intake of food by taking smaller or larger forkfuls therefore eating less or more overall with very little noticeable difference for the body.
Keep some treats in at all times. In the early stages this will be more frequent than later on, which looks strange but if you don't do it this way you will not succeed. In time you won't want the fast food nearly as often anyway and treats may just be a scoop of really top notch ice cream or similar.
