How to find out valency of an element through its atomic number?

The atomic number by itself won't tell you a thing about the valency of an element.  Atomic number tells you the charge on the nucleus, and while it also tells you the number of electrons in a neutral atom, it tells you nothing about their arrangement, nor the number of valence electrons.

But when the elements are arranged in the periodic table, you can determine the number of valence electrons from the position of the element in the table.  The Roman numeral at the top of each column in the periodic table tells you the number of valence electrons.

The atomic number gives the proton number, and the proton number is equal to the number of electrons. For atoms 1-20, you can simply remember that the first energy shell capacity is 2, then the second is 8, then third is 8, then 18 and so on. So for example, if we take Mg (Magnesium), we know that its atomic number is 12. Therefore, the basic electronic configuration is 2, 8, 2. The last number is most important, and that is how many valence electrons it has.

Note: the electronic configuation of Magnesium is formally 1s2 2s2 2p6 3s2, or simply [Ne] 3s2.

It's very easy and simple.
1st pt. No. of electrons in last shell should be 8
2Nd Use 2n*n where n is the no. of the shell K,L,M etc. this formula is used to find no. of electrons that can be accommodated  in a shell
For ex if you want valency of element with atomic no.34
Then Using the formula
For 1st shell 2*1*1=2
For 2nd shell 2*2*2=8
For 3rd shell 2*3*3=18
Here's the trick adding the electrons in these shells.=28 And the no. left is 6
So as we know we need 8 electrons in last shell so, 2 electrons are less so the valency is -2
if the atomic no.were 36 it would be a noble gas valency=0, 37 then a valency =+1

it would be easier if you know the electronic configurations of your elements.
the EC(electronic configuration) of elements tells you the distribution of electrons in their respective K,L,M and N shells, each shell can hold a maximum of 2xnxn (2 times n squared) electrons where n is 1 for K, 2 for L and so on.  
for example:
to find the valency of Sodium of atomic no. 11, all you have to do is write down its EC like this
2,8,1 .. as you see the outermost shell has 1 electron (out of a maximum possible of 18 electrons) which gives you its valency. The sum of the EC gives you the atomic no. ie 2+8+1=11
however
taking another example say calcium of atomic no.20
its EC would as you think be 2,8,10
but its actuall 2,8,8,2.. to be safe forget that lest you be confused and lets come back to the 2,8,10 configuration. its valency is not "10". the valencies will only go uptill 4 eg carbon . to find the valency of calcium reduce 8 from 10 and you get 2 and hence the valency.
another eg sulphur atm no. 16. EC=2,8,6. its valency is 8-6=2
hope this helped

You should know s,p,d,f orbits. I dont think they teach this at class 9. so wait till they teach you this.