Airplay on iPhone

[aaj1]

I need advice on how to use my iphone 4 to control my music library on iTunes.I simply want to play music on my iphone on my home stereo. My set-up is as follows: iMac; airport extreme router (synced to iPhone); airport express connected to stereo amplifier - wireless to computer. All works well playing from computer to speakers when controlled by itunes on my computer but I can't figure out how to configure to allow my iphone to control audio.
can anyone tell me what I am doing wrong?
thanks
al

 

[darster]

On iphone, double click home button, slide bottom bar off to the right. Click on airplay icon and turn airplay on. Then you will be able to play your music from iphone.