First read what all you need to give input to construct diamond unit cell. Whether you want bulk or cluster? These thing you need to decide. Then after the optimisations of the diamond are done, you insert the Nitrogen vacancy.

http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html#__top__

Thank you Vipul. I want to  cluster. I already know the parameters  of  the unit cell for diamond.  But  i don't have any experience in Quantum espresso. Moreover, i don't know how to optimize the structure in espresso. Could you suggest me any example?

Thank you again. 

For optimization in QE you have to specify calculation='relax' in &CONTROL. Additionally you have to add the tag &IONS , where you can further specify your optimization parameters. But you can leave it empty as well. For example:

&control

calculation ='relax',

prefix='diamond',

nstep = 500,

pseudo_dir='whereever_you_have_your_UPF_files',

outdir='./'

/

&system

[ADD OPTIONS]

/

&electrons

[ADD OPTIONS]

/

&ions

/

ATOMIC_SPECIES

C ...

and your cell parameters (if necessary) and atomic positions as well as you k-grid.

The options can be found at the link Vipul Ghemud send to you.

Thank you all. I modeled the Diamond and optimize the model according to your suggestion. Now in order to introduce defect and a vacancy in diamond unit cell could i use xcrysden? How can get and input the coordinate of Nitrogen  and vacancy in espresso. For your kind consideration i have attached the code as below: 

&control

calculation = 'relax'

restart_mode='from_scratch'

prefix='diamond'

tstress = .true.

tprnfor = .true.

outdir = '/

pseudo_dir = ''

/

&system

ibrav= 2, celldm(1) =6.6, nat= 2, ntyp= 1

ecutwfc =110

/

&electrons

diagonalization='david'

mixing_mode = 'plain'

mixing_beta = 0.7

conv_thr = 1.0d-8

/

ATOMIC_SPECIES

C 12.011 C.pz-vbc.UPF

ATOMIC_POSITIONS

C 0.00 0.00 0.00

C 0.25 0.25 0.25

K_POINTS {automatic}

4 4 4 0 0 0

There are several steps to consider before you continue:

For adding atoms or removing them, you might need a supercell. Within this supercell (e.g. 2x2x2 times your unit cell, maybe even more) you can add and remove atoms simply by either placing an atoms inside the cell according to your cell parameters (thus calculate the position of the new atom and add the coordinates into your ATOMIC_POSITIONS; be careful to adjust nat as well as ntyp) or delete an atom from you input.

To create a supercell, you can use xcrysden. Just open your pwscf inputfile with xcrysden, increase the displayed number of unit cells (Modify -> Number of unit cells drawn) and save the structure. From there you should be able to create your input-file and the vacancy position(s) for your N atom(s).

I hope this helps you.

Thank you so much  Kai Trepte. It was really helpful.