How do I reset the initial password on an iMac running Lion?

New to forums. Not entirely sure how this works. But, when I set up my iMac, I created one ID that is required every time I let the computer go idle or when I shut down the system. Now I want to change that ID. I tried by going into Settings and User/Groups and went through a process that looked like it would reset the password. But, when I reboot my Mac, the system takes me right in--right to the desktop. No ID or password required. Can you change your initial password?