Q: I don't seem to have an "Internet Tethering" function on my recently purchased I phone 4 S so how can I use it as a modem

You must be using a supported carrier that supports the Personal Hotspot feature. You must subscribe to a data plan that permits tethering, and your account must be properly provisioned for an iPhone.

Check this list to see if your carrier supports it: http://support.apple.com/kb/ht1937