Diet and Fitness Help

I'm a 20 year old college student who has been hanging on to the "Freshman Fifteen" for about 2 years now. I work in construction so my diet is complete crap, and I loathe eating food from home. I've always been a "chunky girl" because of my short height (4'11) but I've always been able to retain muscle for long periods of time without going to a gym or doing any type of exercise. I'll be going back to college in 3 weeks and will be quitti g my job in construction, I'd love to lose between 15 and 30 pounds by my 21st birthday in Febuary. Any advise? (current weight 145lbs.)