i have mandrake linux.i tried so many times to create bootable cd.but i am unable to find how to make boot cd.so please any one help me...

many cdburning software programs will make one for you. This is from the mkisofs man page.

-b eltorito_boot_image
Specifies the path and filename of the boot image
to be used when making an "El Torito" bootable CD.
The pathname must be relative to the source path
specified to mkisofs. This option is required to
make an "El Torito" bootable CD. The boot image
must be exactly the size of either a 1200, 1440, or
a 2880 kB floppy, and mkisofs will use this size
when creating the output iso9660 filesystem. It is
assumed that the first 512 byte sector should be
read from the boot image (it is essentially emulat_
ing a normal floppy drive). This will work, for
example, if the boot image is a LILO based boot
floppy.
If the boot image is not an image of a floppy, you
need to add one of the options: -hard-disk-boot or
-no-emul-boot. If the system should not boot off
the emulated disk, use -no-boot.

Create a bootimage from a floppy disk, or copy one from a cdrom.
dd if=/dev/floppy of=/boot.img

Have the file system structure you wish to have on the cdrom in a subdirectory along with the boot image. Let's say you have it all in a directory called /cdromimage and you put the boot.img file in the top of that directory.
cd /cdromimage
mkisofs -r -joliet-long -l -o /cdrom.iso -b /boot.img .
then burn the image
cdrecord dev=0,0,0 speed=24 -vv /cdrom.iso