Sometime we want to enter the ubuntu without putting username and password. Actually since 8.10 and newer there is an option during installation whether we want to enter the ubuntu for autologin or not.
But in case you just decide this after you finish installation we still can do the setting to change it. These are the command,
$sudo passwd -d username
Password changed to nothing. Or simply you can jump into the next step though.
Now change the setting in login in gdm & pam to authorize login without password.
1. gdm
$sudo sed -i ’s/#PasswordRequired=false/PasswordRequired=false/’ /etc/gdm/gdm.conf
2. pam
$ sudo sed -i ’s/nullok_secure/nullok/’ /etc/pam.d/common-auth

Restart your ubuntu and it will automatically entering the gnome.