Hey bro,
If you look at the root folder of your theme, you will notice a file called favicon.ico. Create your icon in any program you like (Adobe Photoshop, Illustrator, etc) and then save it as a PNG. Go here:
https://www.favicon.cc/
It will compile and save the icon in the correct format. Overwrite the one you have in your themes folder and voila.. .you have your icon working.
For mobile devices like the iPhone you can save the same graphic from your design program and follow the instructions here:
https://www.cutelittlefactory.com/tutorials/how-to-create-an-iphone-icon-full-workflow/
