How can I connect my i-phone4 to my hifi?

Firstly, excuse my ignorance about new technology.

I've recently got an i-phone4. Ultimately I want to have all my music via i-tunes. I want to be able to play it through my hifi. It's been suggested to me that I use ATV for this purpose. So, I'd link the TV to ATV (via a 'split' cable, yet to be acquired, as I've only one HDMI slot and that's taken up by Sky tv) and link the ATV separately to my hifi.

Is that right? Will the i-phone detect the existence of ATV - how does that work?

Any other ideas are welcome!

Many thanks.