Hello I have webcam with inbuilt mic but I don't know how to use mic?

Not sure what OS you're using, but most likely you should check your sound settings, (record, playback, sounds, mic... etc.) Right click the little speaker icon on the bottom left of your task bar, or, go into your control panel under sound settings. Make sure your device is recognized by your computer and enable them from the control panel and set the levels. Hope it works for you! :)