Q: I restored from Time Machine and forgot my login password

My iMac was stolen in June.  Luckily, I had a fresh Time Machine back up of the system.  I've finally gotten enough money together to purchase a Mini and display to replace the iMac.  Today, I oaded the backup image to the new machine.  Unfortunately, the login password I've always used will not let me log into the system.  How can I unlock the password and finally get into my files, etc.?