Start by making sure you have the latest iTunes. (If you're on a Mac, you will also need at least Snow Leopard.) Plug your iPhone in until your device is recognized, and in the Summary tab, make sure you have checked "Enable Wi-Fi syncing on this device." Now unplug your device and it should still show up in iTunes.
You can now either manually start a sync from Settings/General. Or just plug your iPhone in to charge, and the sync will occur automagically.