In case you want to wear an Artemis costume, because she is the goddess that you admire most, here are the basic element of such an outfit:
Of course, the basic element is a peplos, but in her case it must be knee-length. The Diana of Versailles statue has a very nice outfit, for those of you who are good at draping a piece of cloth it should not be too difficult.
Being a huntress, she also needs a bow and arrows (but please be very careful with such items when you go to a party, think safety first - no, I don't think you'd take the real items, but I know that little children can make deadly weapons out of the most innocuous items).
She was also a goddess of maiden dances and songs, that's why she was sometimes represented with a lyre (and that should not present a threat to others' safety).
Don't forget a nice pair of sandals, that's what I found on Amazon.com (the second pair is for those who don't care that a huntress could not really chase her prey in high heels, the third pair does not have very good reviews, but you could take a look at how it's done and maybe accessorize another pair of flat sandals with a golden ribbon):
Last, but not least, the crescent moon on the goddess' head. Even if in ancient times she was not represented with a moon, the painters and sculptors of the Renaissance added this accessory. But, for the sake of a great Artemis costume, I think you shouldn't be too concerned about historical accuracy! For inspiration for a crescent moon tiara, go to the page with goddess Diana pictures and scroll down the page, you will find some paintings which might help you.