I would like to have Comcast be my default mail on my MacBook. How to I change the Mac. To Comcast?

Isn't comcast an internet supplier? Need more info;
Are you talking about which email client you wish to use, I suppose when you click on the envelope icon in internet explorer? Or am I misunderstanding you.
In internet explorer go to tools, internet options, programs, and then select outlook express, etc.

When I want to email something from a website or from the MSFT Word that I got for Mac, it sends it from Mac mail. I would prefer to send it from my Comcast or Yahoo accounts. I get around it by forwarding the mac mail to one of those but would like to not use mac mail at all. I am new to Mac and didn't know I had Internet Explorer or Outlook Express. I appreciate your help.

Tell me about your mac, what operating system do you have.
Upper left corner, apple icon, "about this mac" tells you os. What web browser do you use.
Do you use safari, (it is a web browser on os 10.0 and later)
If you want to use your comcast account and if the cable guy did not set up your email
You may have to call comcast and ask for internet tech support; they can take you through it. (because you might have to put in some stuff like pop-server and smtp-server info that you might not know right off.) But comcast is an internet provider and usually the installer knows how to set up email for windows but not for mac so you need a technician at comcast who is familiar with MacIntosh computers.
Good luck.