Basic - Internal Micrphone

Make sure your internal mic is enabled using your System Preferences / Control Panel.
Choose this microphone in the broadcasting application as the "Built-in Microphone".

Intermediate - External USB Microphone

For better sound quality, plug a USB microphone into your computer.
Choose your USB microphone by name in the broadcasting application.

Advanced - Studio Microphones and Mixing Board

In a traditional analog environment, use a line-out from your mixing board and route it to the line-in on your computer.
Choose the line-input within the broadcasting application as the "Built-in Input".