It sounds like you created the partition already, but it is NTFS?
I think I would run "gparted" from the installation CD/USB, as it will graphically display your disk layout, and allow you to change it. You will need to choose "sda" from the drop-down list on the right, which would default to the current disk, the CD or USB stick.
Once you have the hard disk displayed, find the Ubuntu partition, and click on it. It should highlight that partition in the list below; note the name in the first column, which will be something like "/dev/sda2", or possibly a higher number. This is the Linux method of specifying disks, because the filesystem is mounted to a directory in "/dev".
You then right-click on that entry in the list, and choose "format to", then choose "ext4". To actually apply this operation, you will click on the green checkmark to the right of the icons in the toolbar, or use the menu "Edit -> apply". This step may not be required, but I'm not sure if you can re-format during the install.
You also need a small swap drive, probably about 5 GB or so. I'd resize the last partition to reduce it by that much, to create a blank partition at the end of the drive. You can also delete the Ubuntu partition, then create a new partition that is 5 GB smaller, and create another after it formatted as swap.
Once this is done, click on the icon on the desktop to install Ubuntu, and when you get to the disk partitioning section, choose "Something Else" for Manual Partitioning. Then choose the partition you formatted. The questions are fairly straight-forward, except perhaps the one that says "Use As", where you enter or select from the lis "/" for the main filesystem, and for the swap drive, you can select "swap" from the list.
I'm probably a bit vague because I don't remember the exact wording. This part is close to the start of the install, so if you get stuck, you can always stop and ask questions.