You need an SPST 5V relay
http://www.miniinthebox.com/electronics-diy-arduino-5v-relay-module_p340896.html?currency=USD&litb_from=paid_adwords_shopping&gclid=CjgKEAjwwPabBRCXo46OtM_RhGMSJACgCeqA2GOpKtXFukaztdkcq0ZIjGLbfw8bNgUyv1JFFCIfcfD_BwE
12v=>COMMON
red wire=>N.O. contact
green wire => N.C.
black wire always connected to ground.
Relay OFF
Valve OFF
Relay ON
Valve ON