The settings for the microphone can be accessed by clicking on the sound icon at the top right bar, which looks like a speaker with the sound waves coming out of it.

If you click on that then you can see the option to open your sound settings. Do this and go to input, which is microphone. You might have a couple options like front mic or rear mic, and you might need to try out those options.

A lot of this has to do with your sound card which is usually on board your motherboard. Can you tell me what kind of motherboard you have?

Oh, and sometimes the problem lies within Skype, try to open the options in skype and see whether you have options for your mic.