how do i make a partition support resizing

I am trying to resize the mac partition using diskutil. I get a message saying that the volume does not support resizing. How do I make the volume support resizing?

here is the command i type:

super-users-computer:~ User$ diskutil resizeVolume disk202 70G

this is what it tells me:

Volume format does not support resizing
Disk Utility Tool
Usage: diskutil resizeVolume [Mount Point|Disk Identifier|Device Node] size
<part1Format part1Name part1Size> <part2Format part2Name part2Size> ...
Non-destructively resize a disk. You may increase or decrease its size.
When decreasing size, you may optionally supply a list of new partitions to create.
Ownership of the affected disk is required.
Valid partition sizes are in the format of <number><size>.
Valid sizes are B(ytes), K(ilobytes), M(egabytes), G(igabytes), T(erabytes)
Example: 10G (10 gigabytes), 4.23T (4.23 terabytes), 5M (5 megabytes)
resizeVolume is only supported on GPT media with a Journaled HFS+ filesystem.
A size of "limits" will print the range of valid values for the current filesystem.
Example: diskutil resizeVolume disk1s3 10G
JHFS+ HDX1 5G MS-DOS HDX2 5G
Valid filesystems: "Case-sensitive HFS+" "Journaled HFS+" "Case-sensitive Journaled HFS+" "HFS+" "HFS" "MS-DOS FAT32" "MS-DOS FAT16" "MS-DOS" "MS-DOS FAT12" "UFS" "Linux" "Swap"

Have you tried to boot from your recovery install DVD and run Disk Utility direct from the DVD? You can not do anything to a hard drive while it's mounted.

I didn't know this. Actually, I don't have an OS disk because I bought this computer on ebay and they didn't have a disk. I'll be installing Snow Leopard and Lion soon. Thanks for your info now I know what's goin on.