You need to have root access to add a user to the sudoers file. So either
su - root
(and put the root password in) and then use
as root to make the relevant and correct changes, or ask your sysadmin.
You usually want something like,
username ALL=(ALL) ALL
%groupname ALL=(ALL) ALL
and put your user ID into that group, but how you configure sudo depends on the standards for your environment.
Changing the sudo config file should only ever be done with
visudo (to prevent being locked out by making a bad change), and it should only be done by someone who understands how to configure sudo safely (personal soapbox: sudo is not the tool some people think it is).