Add the number of Protons and Neutrons.
All atoms are made up of Protons, Neutrons, and Electrons. The atom is made of of the Nucleus which is a clump of protons and neutrons. The number on the periodic table under the symbol is the atomic mass of the element. Protons and Neutrons weigh about the same and electrons barely weigh anything. For example if you have chlorine- 37 you know that all chlorine atoms need 17 protons in order for the element to still be chlorine, and you also know that protons and neutrons weigh the same amount. So you simply subtract 17 from 37. You know that the atom will have 20 neutrons.
TL;DR: Proton + Neutron= AMU= Number under symbol on periodic table.