How do i make my smaller in 3 weeks? I cant fit in my dress HELP!! please please please :(?

I have banquet in 3 weeks i bought the most gorgeous dress. I think my s are too big though to make it fit it sips up until it almost there but it looks like i need to loose some weight or something.Help me please its such a pretty dress i would really like to fit in it. What should i do!!? Do i need to loose weight on my ? How do i do that?