How did you install Ubuntu? Also, is it Ubuntu 12.04 or 12.10? It probably isn't a big deal, but it could be.
Assuming you booted from your hard drive, you're in luck - you have already made a partition, perhaps without knowing it. If instead you meant that you wanted to make multiple partitions here's how:
- Live boot into your system, the same way as when you first installed (disk or flash drive)
- Open a terminal
- Type ´ sudo gparted´
- Look for your hard drive in 'devices' on the right. It's likely to be /dev/sdb but maybe not, so watch out.
- Make sure the device's pprtitions are unmounted (right click them and if it says unmount, do it)
- Your hard drive us likely set up with one large partition and one swap partition. For the purposes of this, wipe out the swap partition (look for the delete/remove button)
- Right click your main partion and click 'move/resize'
- Resize your partition to how big you want it to be. Warning: shirt from the RIGHT not the LEFT as doing this could ruin your bootloader
- Select the area now known as 'unallocated slave's right click, and click to create a new partition. Make this partition however large you want it to be.
- Repeat above u til you're satisfied
- Click the green check mark