How can i lose a dress size?

I am a size 4 in dresses, with a 25/26 in waist. I use to be a size 0 with a 20 inch waist. I'm only 5'3 by the way.(full grown, 19) I miss being really tiny, i ate normal, went on long walks, was def. more active ect. But now, I live in a desert and there is NOTHING to do!(had to quit a band because i moved, but i recently started a new one and i need to get back into shape for shows man!:D) I've been eating a crap load of mashed potatos lately, would you say those are one of the most fattening things i could eat? What sort of workouts should i do? Any suggestions would be appreciated, Thanks a bunch (: