The AirPlay Mirroring is available for some products, some of them are:
iMac (Mid 2011 or newer)
Mac mini (Mid 2011 or newer)
MacBook Air (Mid 2011 or newer)
MacBook Pro (Early 2011 or newer)
And as you've said that your iMac is from 2008, I am afraid that you can't use it with it. In case your product is compatible with AirPlay a menu should appear in the taskbar next to volume, wireless, clock, etc.