If you have forgotten the passowrd you have set for your mac, how to reset it?

First, you will need your user name, you can get it from system preference\accounts

Then you just have to reboot your MAC,

Press Apple+S during booting to enter the single user mode
at the # prompt enter the following command
#sh /etc/rc
#passwd yourusername
#reboot

Do not that you will also lose your password keychain, therefore should only use this method as a last resort.