If you already have an image like your company's logo which you want to use for favicon.ico, you can use an online favicon generator like
Dynamicdrive Favicon generator to convert the file format from .gif, .jpg, .png or .bmp to favicn.ico file format. After the favicon has been generated, download it to your local computer and upload it to your server.
NOTE: Your site's favicon file should be accessible by typing
http://www.yoursite.com/favicon.ico