simply googling ... autologin in debian .
I remember that I had the same issue when used Xubuntu and i resolved it and If I recall right , the follow helped me.
Ok. Ubuntu is based in Debian , but is not Debian. But you can try and error !
Can I suggest a friendly advice?
Before asking ,try as I suggested. Googling and try.Only in this way you will learn .
Anyway, you are welcome.