In order to use the iPhone as a modem (to "tether" it with your laptop), you will first need to activate the service on your account with your mobile operator.

When activated, you will have the option of enabling Internet Tethering in the Network Settings menu:

To use the iPhone as a modem via Bluetooth, you must first turn on the Bluetooth service on the device, this is located under the General settings menu:

Once Bluetooth is activated, you can then enable the Internet Tethering:

On the Mac, launch the Setup Bluetooth Device wizard:

The Mac will then search for available Bluetooth devices within range and display a list of those devices it can detect, select the iPhone:

The Mac will generate a random PIN for the Bluetooth connection:

Enter the same PIN onto the iPhone when prompted:

The Mac will install a new Bluetooth network adapter for the connection:

Select the option to open the Network Preferences, the new Network Adapter will be displayed:

To initiate Internet sharing from the Mac, press the Connect button:

After a few moments your Mac will be connected to the Internet via the iPhone's data connection. You can now close the Bluetooth Setup wizard: