How can i lose 30 pounds in a month or 20?what exercises should i do?

Well i am 13 and i am trying 2 lose some wight im not big im thick lol!anyways i have a wedding on aug.14 and first day of school is aug.16 but i need to look good for the wedding bc i am a junior brides maid all eyes are gonna be on me and i need to look perfect next 2 my skinny cousin!So plzz help!