Edited sudoers file with text editor - now can't run sudo

I see you've tagged your question osx so if you've done this on a mac, make use of the GUI.

1. Open any Finder window and press cmd shift G
2. Type /etc/sudoers and press return to go to the file
3. Press cmd i with the file highlighted
4. Scroll to the bottom of that info window to 'Sharing & Permissions' and click the lock icon in the bottom right
5. Type an admin username and password
6. Now add yourself in that window with the + button and select 'Read and Write' privileges
7. Open the file in any editor and fix what you screwed up the first time!