How to not get fat working at mcdonalds?

I just started working at McDonald's and im really concerned about my weight. They give me a free meal when i work but i don't eat the beef sandwiches or touch the fries. I usually get the premium chicken sandwich called the club, crispy and i substitute the fries with apple dippers. sometimes i get a snack sized oreo mcflurry. by doing this regimen, i've gained about 2 pounds in the last month but since track season ended, i haven't been running lately. but how can i eat healthy at a place like that?