Look on the internet for a picture you like (google images) then find the right style dress, jewelry and hair.
Sandals for your feet and lots of eye liner and mascara
It all depends upon which goddess you wish to dress up as... If Hera, Aphrodite, or Athena were your choice, then the answer given by 'Anonymous' would work fine. However, if you were to chose Artemis (the Huntress), you would definitely NOT wear make-up, save as to make your skin appear fresh, clear and clean. You would use a bronzing powder to make you look lightly tanned (from all the time in the sun, hunting), the clothing would usually be leather and metal armor (feminine) or a loose garment, easy to move in, and you would carry a bow or hunting spear.
Look up images of the Goddesses you are interested in, there are more different types than some think!
Good Luck, and I hope that helps!