Depending on your body shape, a little black dress would ideal for all body sizes. You need also to look out for jersey fabrics that give a little but also hug your shape at the same time. A bright blazer on top of your dress would also work out best. Make sure to get the right size for your bras and panties. Learn more from http://www.doctoroz.com/videos/slimming-wardrobe-secrets.
Cut certain fatty foods, like dessert and fast food, etc. For motivation, just think of being the size you want by the end of a certain amount of time! It'll help you choose good foods and excersize. Hope this helped! :)