What should i wear to a costume party?

Now im brownish and tall, 14 and a little skinny but with more muscle than most my age... and i don't know what to wear!!!!!!!..... I was thinking of doing the terminator but im not so sure, and i need a backup plan. help me, its gonna be like a birthday party for my girlfriends friend, and my ex is gonna come. I don't wat to look silly but i have no idea of what to wear, HELP ME!!!