To make a call using your Bluetooth headset, you simply dial the number as you normally would. (If you're using a headset that accepts voice commands, you may be able to dial by voice.)
Once you've entered the number to call, your iPhone will present you with a list of options. You can choose to use your Bluetooth headset, your iPhone, or the iPhone's speakerphone to make the call.
Tap the Bluetooth headset icon and the call will be sent there. Now you should be connected.
You can end the call by using the button on your headset, or by tapping the "End Call" button on the iPhone's screen.