Creating directory on an external hard drive via command line (SSH) on Linux

I have tried many approaches to create a directory on my external hard drive. Here is the details when I do

fdisk -l

What I get:

[email protected]:~$ sudo fdisk -l

Disk /dev/mmcblk0: 3965 MB, 3965190144 bytes
4 heads, 16 sectors/track, 121008 cylinders, total 7744512 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: 0x000a589c

        Device Boot      Start         End      Blocks   Id  System
/dev/mmcblk0p1            2048     2464843     1231398    e  W95 FAT16 (LBA)
/dev/mmcblk0p2         2465792     7744511     2639360   85  Linux extended
/dev/mmcblk0p5         2473984     2588671       57344    c  W95 FAT32 (LBA)
/dev/mmcblk0p6         2596864     7744511     2573824   83  Linux

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: 0xbe707a93

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *          63   625137344   312568641    7  HPFS/NTFS/exFAT

I found that my external hard drive is most likely located at /dev/sda so I tried 'cd'ing to it, but came to a dead end when it says

[email protected]:~$ cd /dev/sda

-bash: cd: /dev/sda: Not a directory

Any help with getting on with my task?