What are you trying to calculate? Structures, properties, or just energies? Either way Quantum Espresso is an example of a free to use package, and has a good forum for support.

Dear Vinod,

It is dependent on which physical or chemical properties that you want to calculate.

As reza said PWSCF (Quantum Espresso) is a free suitable code for calculating electronic, elastic and magnetic properties of materials.

I am not massively experienced with DFT, but have used CASTEP before which I believe is free for academic institutions. But as others have said, it depends on what you want to calculate.

Thank you @ christopher , jaafar and Robert for your response, I want to know the change in the electronic structure on doping with univalent cation in ZnO