You need two things in order to calculate the average atomic mass of chlorine (or any element for that matter): the exact weight of all its naturally-occuring isotopes, and the abundance of each of those isotopes.
In chlorine's case, you have two naturally-occuring isotopes, each with a different mass number - chlorine-35 and chlorine-37.
The exact weight and the abundance of these isotopes are
The procedure is very simple - you just multiply the weight of each isotope by its abundance and add the resulting values. I'll use decimal abundance, which expresses the abundance as numbers between 0 and 1, for simplicity.
To get decimal abundance, just divide the percent abundance by 100.
So, the average atomic mass of chlorine will be
The average atomic mass of chlorine is 35.4527 amu.