Hard-drive partitioning AFTER Ubuntu 12.04.1 LTS installation

Question

How can I partition my hard drive (so as to share files between Windows 7 and Ubuntu 12.04.1 LTS) AFTER I've already installed the Ubuntu OS on the initial NTFS hard drive (which has allowed for dual boot capabilities)?

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@Vasa1

johnmellin@ubuntu:~$ sudo fdisk -l
[sudo] password for johnmellin:

    Disk /dev/sda: 320.1 GB, 320072933376 bytes
    255 heads, 63 sectors/track, 38913 cylinders, total 625142448 sectors
    Units = sectors of 1 * 512 = 512 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disk identifier: 0x8d769ec6

   Device Boot      Start         End      Blocks   Id  System
    /dev/sda1   *        2048      409599      203776    7  HPFS/NTFS/exFAT
    /dev/sda2          409600   598849535   299219968    7  HPFS/NTFS/exFAT
    /dev/sda3       598849536   625139711    13145088    7  HPFS/NTFS/exFAT

johnmellin@ubuntu:~$ sudo parted -l

    Model: ATA Hitachi HTS72323 (scsi)
    Disk /dev/sda: 320GB
    Sector size (logical/physical): 512B/512B
    Partition Table: msdos

    Number  Start   End    Size    Type     File system  Flags
     1      1049kB  210MB  209MB   primary  ntfs         boot
     2      210MB   307GB  306GB   primary  ntfs
     3      307GB   320GB  13.5GB  primary  ntfs