Open your terminal and type this

sudo apt-get install gparted

then click at unity dash and type as gparted . it will gives you gparted application icon , click it to open . it will prompt for your root password nothing but login password(if you are singe user ) .

it will display you all the partitions and empty space of your HDD . Now right click at the unallocated/empty space . among the options click at NEW . then it will open the window . give the required size and type of the partition .

look at the image for more information .