All you need is a "line out" and a cable to conect with.
I know of two ways to get the line out. Either with the Apple Dock, or the Sendstation "PocketDock". You technically can use the headphone out, but you won't be satisfied with the sound quality.
I have the Monster "iCable" which is a cable with an 1/8" jack at one end, and RCA plugs at the other end. There are other brands that are cheaper, but I feel quality cables do help with sound quality.
You should be able to plug in place of your CD player or Auxilary input, but avoid "Phono", there always seem to be audio problems attached to that one. Good luck.