A good rule of thumb you can use is if you don't know whats in the food (eg all the chemicals) then don't eat it! No it wont poison you but I'd bet your body can do without the additives and quite often additives come submerged in fat, refined sugar and various chemicals.
If you can, see if you can get stuff steamed or grilled rather then fried, roasted or boiled. Little fat is used when grilling and none when steaming.
I would also try to get stuff pared down- add flavor with your own sauce rather then foods which come in sauce. Potatos alone and add ketchup if you really have to. Unsalted and then salt yourself, and if your given the option of two sorts of carbohydrate, try to go for the darkest option- these tend to be less refined, keep you feeling full for longer then the refined version and if stuck to chose between two forms of protein, go for lighter/whites over darker colours. The darker versions are not really that bad but sauces tend to get added and cream based sauces are normally more calorific and higher in fat then the darker gravy or tomato based versions.
If you can, try to make and bring your own. It will save you so much stress! Maybe if you have to eat food cooked for you, try to limit it at once or twice a week. Once you start cooking for yourself it will get easier the more you do it.