Look
Your sound card has a microphone imput. You plug it in there. (usualy its color is pink whilst the speaker color impit is green)
Than you set it up so it works in windows using the wizard. When in ESF go throo your key configuration. Youll find voice_chat (or something like that) remember that button.
Than at the voice menu of the options in ESF. Enable the voice and set the volume.
Ingame to speak you simply press the voice_chat button and talk into the microphone. When done let go of the button.
But mics are normaly anoying in ESF ^^