How to add user as sudoers

A few minutes ago I can't install new software in Ubuntu 14.04. It said that my user is not in the sudoers file (or something, I forgot what it clearly said in terminal). So here we go..

1. Open terminal
2. type su -
3. type your user password
4. type sudo adduser your_user_name sudo
5. just it. Hope this help :)