Ubuntu14.04 installed but no GUI

I have windows 7 64 bit on my system, I installed ubuntu 14.04 with help of daemon tools. everything runs smoothly and while booting of ubuntu , i seen a /tmp directory issue which ignore withI

now I am getting only tty1 to tty6 screen , no GUI. I tried to install unitiy using

sudo apt-get

but gives error unable to mkdir /var .. ( I don't remember complete sentence )

I found that /root doesn't have write permission as well as /var directory

Please help me to enable/install GUI screen.

what i think :

I have to mount that image which may be essential to complete some missing things.