This expert is helping you out because there weren't many active Comcast experts.
The easiest way is to first go to xfinity.comcast.net. Depending on which browser you use, the next step will vary a little:
Internet Explorer - go to tools, then choose internet options. On the first screen you come to, you will see settings for your home page. Look for the button marked "use current". Your home page will be changed to Comcast.
Safari (Mac): Click on Safari in your menu bar and choose Preferences. On the General tab, you will see "Homepage" with the current address. Below that is a button marked "use current page". Click and you're all set.
I have an MCSA cert. I have been in the IT/internet area for over 9 years. I have had many experiences in the IT field and have been in many different areas of the field. I am no longer a Comcast employee and will gladly tell it like it is to anyone. If you are not happy with your service, there are many other providers out there.
Simply go to the page you wish to have as your homepage, when there, click on tools, then options, then select the option that says "USE CURRENT", then click on OK. This will set that as your homepage.