The superuser is a privileged user with unrestricted access to all files and commands. The superuser has the special UID (user ID) 0. You need to become super user (root) only when tasks need root permissions. Here is how to become super user:

1) At shell prompt type su and press enter key, when prompted for password supply root user password:
$ su

2) To exit super user status type exit or press CTRL+D
# exit>

Please note that if you get an error su: Sorry for normal user account. Following workaround needed to get rid of this problem/error:

1) For security, reason FreeBSD only allows su to root user, if user is member of wheel group. Wheel group is a special group for administration purpose. Add your normal user to this group using pw command using following:
# pw user mod username -G wheel

2) So to add user vivek to group wheel run command as follows:
# pw user mod vivek -G wheel
# groups vivek

vivek wheel

3) Now su will work for vivek user.

You can disable this behavior complete for all users (not recommended until and unless you trust ALL of users):
1) Open pam configuration file for su using text editor:
# vi /etc/pam.d/su

2) Look for following line and comment it out:
auth requisite no_warn auth_as_self noroot_ok exempt_if_empty
Replace with:
#auth requisite no_warn auth_as_self noroot_ok exempt_if_empty

3) Now all users can use su command.