don't buy foods that are processed. if it comes in a can or bag, avoid it if you can. stay away from fried foods and foods that have a lot of sugar. drink mainly water.
start by removing all of the food you have that is fried. get rid of the chips and nachos and frozen foods like breaded appetizers and chicken fingers and french fries.
Once you've tossed all your fried crap look at all of your sauces and condiments in the fridge. If anything is creamy, chuck it. No alfredo, ranch dressing, mayonnaise, etc These things are really really high in fat and calories and they make your good food bad for you.
After that, hit the pantry and look at all of your canned goods. If you have tins of beans and lentils and such, that's great. If you have pre-made soups or chili or whatever that's bad. That kind of food is highly processed and is full of salt and chemicals and it is not good for you.
The next time you go for groceries, stick to the outside perimeter of the store. Avoid the rows as best you can. The food on the outside is 'real' food. Veggies, dairy, fresh breads, etc.
The best rule of thumb is to never eat food with more that 5 ingredients listed on the label. If there are more than 5, then that food is no longer food. It's chemical garbage.
The next thing you need to do is go to WeightWatchers and learn portion control and learn how many calories (points) you are allowed to eat.
Stick with your eating plan, go to WW meetings, stop eating processed garbage.