Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
You turn on personal hotspot, you have a choice of connecting via USB or wifi simple
 
If you connect your charger cable to your iPhone and then plug it into your computer the iPhone will act as a USB modem if hotspot is turned on
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.