Your iPod touch will not be able to do anything over bluetooth with your iMac. Bluetooth is useless on the iPod touch and iPhone if you wish to use it to "sync." It is non-functional and a dead end to pair your iPod touch with your Mac over bluetooth, and you cannot use a bluetooth connection with your iPod touch for network/internet access with your iMac.
If you're trying to set up your iPod touch to connect wirelessly and be able to access the internet, then you want nothing to do with bluetooth -- you want wifi instead. Whether or not this involves your desktop iMac at all depends on how your wireless network is set up.
* What kind of internet service are you using -- DSL? Fiber? Cable?
* Are you using a wireless router, by LinkSys, D-Link, Belkin, Apple, etc.?
* Is your iMac connected to the internet via a wireless connection, or a wired connection (ethernet)?