1st option
Try to log on on an another admin account on your Machine and change that.
2nd option
if there is no other admin you have to create a new user as an admin.
Reboot. When the screen goes black hold "command- s" to enter in Single user mode
type /sbin mount -uw /
its /sbin(space)mount(space)-uw(space)/
then enter
type rm /var/db/.AppleSetupDone
its rm(space)/var/db/.AppleSetupDone
enter
Type Reboot
enter
The system, will restart with like setting up a new machine (dont worry your user files are not deleted)
set up a new account. then after that check teh user accounts on system prefrences. remove managed and boot on to your pervious user.
After managing to make your Previous usre an admin delete the other extra user that you created
Try to enable firmware password so that you can only do that to your machine. http://support.apple.com/kb/HT1352
Turn on Filevault to protect your files. http://support.apple.com/kb/PH7033