Can anyone tell me how I can back up all of my stored e-mails, bearing in mind I am computer illiterate.?

As I have stated I am computer illiterate and a friend will soon be trying to make my computer operate a little faster by going back to zero, I apologise if I am not using the correct jargon. This I am told might mean I lose all of my stored e-mails which I do not want to happen. I would be grateful for any guidance, given at a basic level. Thank you.