Ubuntu – How to use gparted to resize hdd

In order to resize your partition you'll need to unmount the disk. Though it's likely that your trying to edit the same partition of the drive you booted from. In order to resize that drive you'll need to boot from an Ubuntu live disk and edit the partition from that.

You cannot resize a partition while it is in use (mounted). Hence its greyed out.

To resize your partition, you would need to boot from your Ubuntu 12.04 LiveDVD,

open Gparted, and then resize your Ubuntu partition.

Depending upon your current partition setup, you may want to shrink different partitions

If you have a large home partition (/home), you may shrink it by 100 gb, and in the free space created, create a ntfs partition which you could use for Windows.

Post your output of

sudo fdisk -l

so that I could better advice you in partitioning.

Edit-

As you stated that you only have single partition, just resize (shrink) it by 100 gb.