how to format mmc card...

i hav an external sd/mmc card reader...
my mmc card is corrupted...
is there anyway to format it in knoppix?

If this is a card for a camera or some such you can probably format it with the camera. If that's not possible do first fdisk -l to check the formatting.
You can format it with mkdosfs -F 12/16/32 /dev/sd?? (choose FAT12, 16 or 32 and check the device from fdisk output)

But how to format a USB mass storage device to FAT12?
Please help me. :(

first you have to know what the disk is called under linux.
if you type sudo fdisk -l in a console (konsole, xterm, aterm... they are the linux equivalent to dosprompt) you will get all detected disks and their filesystem type listed.
Or you cold try clicking on the "usb dongle" icon/s on the desktop. you then get a filemanager showing the content of the disk, one of those should be your device, look what it is called and use that.

let us say that /dev/sdb is the one you want to format. then you do sudo mkdosfs -F 12 /dev/sdb