There are some tethering apps out there.
Only if it is jailbroken, I believe. If you are jb, look up TetherMe or MyWi.
If not jailbroken, your option is to contact AT&T and add a Tethering Plan to your account, at last check I think it is $50/mo for a 5gb plan, month to month. Or change your plan into the Data Shared plan and get Tethering for free but it goes towards your monthly limit of GBs.
I got FlashArmyKnife before it got pulled from the app store