Of you've been eating shitty, be prepared for it to suck for the first few days. If you want a simple and cheap way to start out, here's my general meal plan:

Breakfast: a piece of fruit, or oatmeal without sugar, or a smoothie with a fiber supplement added to it.

Lunch: Broccoli, carrots, Celery, cucumber, peppers, radishes, etc. - basically any veggies you prefer and frankly as much as you want, they're so low calorie that you can afford to fill up. Use hummus as a dip, or find a salad dressing that doesn't have added sugar or high fructose corn syrup. Oil based is fine, fats > sugar any day. I get those little 2oz disposable cups with lids and take it in that so I don't end up using too much.... If you're using hummus, you can have much more than the dressing just don't go overboard.

Dinner: I find it easy to stick to a plan if I don't have to cook it- so pick up a rotisserie chicken and some frozen vegetables that you can steam in the microwave or saute with some spices. Eat about 1/4 of the chicken and a whole bag of veg, like these.

If you want a snack or are craving something sweet, an apple has enough sugar to satisfy that usually, but you could also go for Artic Freeze (or whatever it's called) ice cream alternative. They're like $5/pint so it's not cheap but if you absolutely can't live without ice cream, it's worth it. It's water based with whey protein so you can eat a whole pint and it's only like 280cal.