How to make a fav icon:

Create an image for each size you want to provide an icon- 16x16, 32x32, etc.

Here's a great site that will create fav icons from your image.

http://www.favicomatic.com/

How to add a fav icon to your site:

Add a link to the fav icon to the head of your index.html. For example:

<link rel="icon" type="image/png" href="Content/images/favicon-32x32.png" sizes="32x32" />

Learn more about fav icons:

https://en.wikipedia.org/wiki/Favicon