Semi-Formal!! Help

Oka .. my semi formal is coming up next month and Im a grade nine student so I dont know what to wear!!
Heres some facts: Im 5'1, brownish hair thats about a inch and a half below my chin, i weigh about 105 . .
Please Help Right Away...
(p.s. to any guys reading this what would make yu take a second glance at a girl at semi!?)

For all my semi formal dances I just wore a nice pair of pants and a cute shirt. Thats pretty much what everyone else wore too.