Edit: Cross-posted with Alex. I'm writing this one as a 'reply', so both answers will look consecutive.
Do you know how to add possessives "my, your" etc? You usually add "-m" (For 'my') ; "-n" (For 'your') ; "-sı" (For 'his').
Like: Araba (car)
Benim arabam ; Senin araban ; Onun arabası.
It's the same thing with "arka". It means "the back side", and it's a noun.
Benim arkam (my back side)
Senin arkan (your back side)
Onun arkası (his back side)<pre>
Benim arkamda = behind me (at my back side)
I think you mean the article :D
When a subject, there is no way to tell the difference between the two in Turkish. You can optionally add "bir" to really make a difference, but its use isn't required.
Now as a direct object, you will see the accusative case pop up when you are talking about something speicfic (the) and no case when you are talking about something general (a)