Unable to remember iMac password

I am trying to set up my second (old) iMac for my kids to use. this has not been used for a year now. When switched on , asks me to sign in. I can not remember the password for this particular imAc. The system asked me to sign in using my account id and password. but it keeps coming back to same message, ie 'sign in using account"

[Re-Titled by Moderator]