First you'll have to give up your unlimited data plan and switch to one of the new mobile share plans. Then you'll need to add tethering to your plan. Then you'll be able to activate the hotspot on your phone.
You cannot use the hotspot feature with the unlimited plan. The use of the hotspot has to be supported by the carrier, and for AT&T you have to have one of t he tiered data plans with tethering support added.