Protons = Atomic #

For most uncharged atoms, the electrons equals the protons.

The neutrons are what varies. You often have various isotopes of the same element. However, on the periodic table you are given an atomic mass or atomic weight which is the mass of the nucleus. Therefore, if you subtract the atomic number from the atomic weight, you will get a good idea of how many neutrons you should have.

For example, you have Hydrogen-0, Hydrogen-1 and Hydrogen-2 (a radioactive isotope). Most common is either zero or one neutron (the number after the element name is the number of neutrons).

If I remember correctly, electron shells are 2, 8, 18 for the first three.