1.Create a new partition
sudo fdisk /dev/sda
This will bring up the fdisk menu, you should familiarize yourself with it, search for tutorials (here’s one). The basic options are:
a toggle a bootable flagL
b edit bsd disklabel
c toggle the dos compatibility flag
d delete a partition
l list known partition types
m print this menu
n add a new partition
o create a new empty DOS partition table
p print the partition table
q quit without saving changes
s create a new empty Sun disklabel
t change a partition’s system id
u change display/entry units
v verify the partition table
w write table to disk and exit
x extra functionality (experts only)
If all goes well, by pressing N, you will be given the option of creating a new partition in the empty space. If the only unallocated space is the one you want to use, the default choices should be fine and you can just let fdisk choose.
2.Since you want to create an ext partition, you don’t need to do anything. If you wanted to create a different type (swap or NTFS or whatever) you would need to use t to change the partition type. I am mentioning this just in case.
3.Check that your changes are what you expected by hitting P to print the partition table.
4.If everything is OK, write the new partition table to the disk (W) and exit (Q).
Now that you have your new, empty partition, you can create it’s filesystem. So, if you just created /dev/sdaX (where X is the number of the partition you created, for example /dev/sda2), run this:
sudo mkfs.ext4 /dev/sdaX