fsck? ... unexpected consistency... ???

[yellowdog-general@lists.terrasoftsolutions.com]

I received the following error message while booting Linux 2.4.19 on 
2001 ibook.

/dev/hda12/ contains a file system with errors, check forced

...

/dev/hda12: Unexpected Consistency; run fsck manually (i.e., without -a 
or -p options

[FAILED]

what? how do i run fsck? i typed fsck and it prompts me to a line but i 
dont know what to type.

thanks for any help.