How can I remove all Mail accounts and start over?

A neighbor has asked me to help him redo his email; he currently doesn't receive any. His platform is an ~ 8 year old Macbook Pro. i am a PC type, but know enough to be dangerous from using my iPhone. I've no experience on his Macbook though.

He is not a tech savvy person, and the best description I can get from him is that he quit receiving email "when I switched from Verizon to Comcast." His Comcast account is active, as he is connected to the Internet. I am assuming his username and password for that Comcast account are retrievable, as well as his Apple ID and password.

My plan is to completely wipe out any email settings, accounts, etc. currently on the MacBook, and proceed from scratch. I think, from YouTube videos, I can set him up properly. But what's the recommended way to wipe out whatever is there now?

[Re-Titled by Moderator]