if you've goofed around and tried to change your user name though the advanced settings in your mac, you may have ended up with your user switching back to a standard account. if you only started with one account, then you've left yourself with no admin account. Aaaand you can't change anything. Here's is what you do:
reboot your mac
hold command-S before you hear the chime
you'll see a bunch of verbose text fly past. when it's done...
enter: mount -uw /
enter: rm /var/db/.AppleSetupDone
hit control-D on the keyboard to restart.
Your mac will now boot and start the setup wizard again. Go ahead and create a new admin user.