You've probably already heard this but a good place to start is reducing your or cutting out completely any junk food, and by that I mean foods that tast good, but don't add a whole lot of nutrition to your diet. I know it's hard to keep a nutritional diet ona budget, since things like fast foods and candy bars are easy and cheap to grab. But something as small as replacing a sugary. caloric soda or smoothie with water can make a small improvement.
Maybe you can start there and work your way up to cutting out more junk food? As Lemon Squezzy suggested maybe keep a food log, so then you are more aware of what you are taking in and burning, and where you can make healthier changes (i.e. rosted chicken instead of fried, an apple instead of a bag of M&Ms, etc.).
I know easier said and done. But for 6 six weeks during lent all I did was cut out all junk food from my diet and I lost about 10 pounds. I know that's not everything that you want, but it's a start, and adding exercise may help with the rest.