Question

How do i find the valence electron for Chlorine.

Answer 1

You look at which column it's in (seventh). that means its outer shell has 7 valence electrons

Answer 2

And 7 electrons in the outer shell means it lacks 1 to make an octet; therefore, it will gain 1 electron to make the valence of Cl -1.
Generally, Group I elements have 1 electron in their outer shell; they lose this one to become a +1 valence.
Group 2 elements have 2 electrons in their outer shell; they lose these two to have a valence of +2.
Group 3 works the same way for +3.
Group 17 elements (or group VIIA depending upon the system you are using) have 7 electons in the outer shell; they gain one electrons for a valence of -1.
Group 16 elements (or VIA) have 6 e in their outer shell and they gain 2 electrons for a valence of -2.
Group 15 elements (or VA) have 5 e in their outer shell; they want to gain 3 electrons for a valence of -3.