add yourself to the sudoers list. To do that you need to switch to the root user account.
Enter your root password
Now that you are root run visudo
What this does is open /etc/sudoers with vi/vim to be edited. This means you will need to know how to use vi/vim to change this file.
Now look for the line that reads "root ALL=(ALL) ALL". Right below that line press the i key to enter insert mode and add "mark ALL=(ALL) ALL". Afteryou have added the line press ESC key and then press shift+:. Now press qw and then press the ENTER key. This should save teh changes and exit the file.
Now you can press CTRL+D to logout of the root session and go back to working as the user mark. Now when mark uses sudo and you type in the password for mark it should work.