First you need to make sure you have IOS 4.2 or later to have the hotspot feature.
- Go to Settings. Select the Personal Hotspot setting
- Switch personal Hotspot to on.
- Note the Wi-Fi password on your phone
Now on your laptop ensure the wireless is on, search for wireless networks and you will see one which is the name of the phone. Choose to connect and enter the password noted above. You will now be able to get onto the internet through your iphone as a tethered modem. Be sure to check your mobile phone plan data allowance to avoid getting hit by an excess usage bill!