After fresh installation of Debian 7.0 wheezy I tried to do sudo su - . But got the first error message
“sudo: command not found” . As per error message it means sudo is not installed in Debian system.

linux@debian:~$ sudo su –
-bash: sudo: command not found
linux@debian:~$

To solve this issue install sudo in Debian,follow the given below steps

Step 1: First you have to become super user (root)

su -

Step 2: Now install sudo with the help of command apt-get

apt-get install sudo

Step 3: Now check the sudo

root@debian:~# exit
logout
linux@debian:~$ sudo su -

We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

    #1) Respect the privacy of others.
    #2) Think before you type.
    #3) With great power comes great responsibility.

[sudo] password for linux: 
linux is not in the sudoers file.  This incident will be reported.
linux@debian:~$ 

Note: user name linux is not listed in sudoers file hence to allow the user called linux to use “sudo su -“. We will edit the sudoers file

Important note: With sudoers file you can also specify which particular commands a particular user or group can use.

Step 4: Allowing user to use “sudo su -”

Because in my system user called linux is first user.I want to give access of super user to this account.

switch to superuser root

linux@debian:~$ su -
Password: 
root@debian:~#

Edit the /etc/sudoers file: Append the below given line in sudoers file.
Replace the user name called linux with your user name. save and exit

vi /etc/sudoers

linux   ALL=(ALL:ALL) ALL

Step 5: Now recheck. Exit from console and login with user and use the command sudo su –

root@debian:~# exit
logout
You have mail in /var/mail/linux

linux@debian:~$ sudo su -
[sudo] password for linux: 
root@debian:~#