i have a linode account and trying to setup an email server where i can send and receive email from my registered domain.
i typed this into shell: sudo tasksel install mail-server
but what is the next step?
Running a mailserver is not a install-and-go process. Over time you'll want to deal with spam and viruses and people trying to hack your machine via the mail servers. Despite hosting dozens of domains, I don't host email at all. I've farmed all of that off to Google Mail and I recommend everybody does something similar.
Life is too short to worry about your email server twice a day.
But assuming you still want to install a mail server, you'll want to follow the proper guide: https://help.ubuntu.com/community/MailServer
The short version is: