Where do I put the Javascript?

Hi there,

i’m customising Twenty Thirteen using a child theme.

I’d like to add a dropdown menu as shown here here..http://tympanus.net/codrops/2012/10/04/custom-drop-down-list-styling/

I don’t have much experience working with javascript and I have no idea where it should go in the theme/pages.

Any help much much appreciated 🙂 xx

Thank you
C