Hi! You've got a clear goal.. that's a great bigining!
..you have to keep in mind this won't be easy, but this is gonna worth it. Then, be concious you need a lot of discipline and willpower.
You can do it!
First, start with some easy things: spend more time in motion and less resting, don't eat more than your body requires, instead of junk food eat a fruit, drink water instead of soda, etc...
Now, besides taking care of your diet, you need to workout. Go from casual workouts to the intense ones according to the parts of your body you wanna focus in.. increase the intensity and time you exercise when you consider it's time to try something harder.
Remember eating healthy, drink a lot of water, and workout at least 30min/day, it sounds very familiar.. but there's no other way you can make this and no affect your health.
I'm pretty sure in 2 months you'll look amazing in that dress. So, go ahead. No doubts, no leaving this for later.. you'll get it.
I'm here for whatever you may need ..Good luck!