1. Reboot
2. Hold apple key + s key down after you hear the chime. (command + s keys on newer Macs)
3. When you get text prompt enter in these terminal commands to create a brand new admin account (hitting return after each line):

mount -uw /
rm /var/db/.AppleSetupDone
shutdown -h now

4. After rebooting you should have a brand new admin account. When you login as the new admin you can simply delete the old one and your good to go after this!