I have found that when I do have a binge out on the high fat things like chips and chocolate it does have a completely disproportionate effect on my motivation. I feel lethargic and I crave more of it and then want to do less and I spiral down.
My rule of thumb is that the more packaging it has the less healthy it is really meaning the more processed it is the more unhealthy it is. Try to get a third of your plate as fruit and veg, another third of carbs like bread, rice, potatoes, pasta, the final third of your plate should be divided into protein (non dairy) and milk/dairy foods. Foods high in fat or sugar should be avoided or very limited.
As for starting to exercise walking is a great start and then start to add in some functional exercise like squats, planks, crunches. There are lots of exercises you can do without spending any money, check out the videos on youtube.