I am first time Please guide me mail server step by step
 

I registered domain

I need step by step setup mail server

sendmail or postfix with squirrekmail

we aren't going to guide you through such a big thing, it's not at all realistic to expect someone to do that.

This might be a good starting point, presuming you know how to generally use Linux already http://wiki.centos.org/HowTos/postfix

If your email users are less than 10.
I think you should consider using GoogleApps email service.
Only thing you need to do is point your DNS google servers.
And after you created an admin account, you can create your email users.
There are tons of tutorials out there, but there is no need.
GoogleApps registration process is very clear and simple, contains instructions about the installation process.
Good luck.

forum is for helping and solving issue if user are stucked .we can't help you on configuring your server step by step.
we can guide you how and where to start...at least you search your requirement in google you will get step-by-step installation on mail server...atleast you try.

http://www.linuxmail.info/postfix-sm...owto-centos-5/
http://wiki.centos.org/HowTos/postfix

try this link.......