CentOS 5
Add a User
[1] Add a new user. I used user name as 'cent' on following examples, but Set any names you like to use.
[root@ns ~]#
useradd cent

[root@ns ~]#
passwd cent

Changing password for user cent.
New UNIX password:
# // input password you want to set

Retype new UNIX password:
# verify

passwd: all authentication tokens updated successfully.
[root@ns ~]#
[2] Try to switch by user that was added in section [1].
ns login:
cent
# input user name

password:
# input password

[cent@ns ~]$
su -
# switch to root

Password:
# input password for root

[root@ns ~]#
# done to switching to root

[3] Make 'cent' user that was added in section [1] be only a user who can switch to root.
[root@ns ~]#
vi /etc/group


# line 11: add a user

wheel:x:10:root
,cent


[root@ns ~]#
vi /etc/pam.d/su


#%PAM-1.0
auth sufficient pam_rootok.so
# Uncomment the following line to implicitly trust users in the "wheel" group.
#auth sufficient pam_wheel.so trust use_uid
# Uncomment the following line to require a user to be in the "wheel" group.
#↓ remove '#' that was on the head of lin

auth required pam_wheel.so use_uid
auth include system-auth
account sufficient pam_succeed_if.so uid = 0 use_uid quiet
account include system-auth
password include system-auth
session include system-auth
session optional pam_xauth.so
Forwarding email for root
[4] Set config to forward mails for root to a user who is a system administrator you set as.
[root@ns ~]#
vi /etc/aliases


# Person who should get root's mail
# bottom: remove '#' and change your user name

root:
cent


[root@ns ~]#
# set new aliases

/etc/aliases: 77 aliases, longest 10 bytes, 776 bytes total