Periodic tables are found online and in chemistry textbooks. They list every known element and provide the number of protons and atomic masses for each element.
The atomic number is located above the element symbols. This is the number of protons found in one atom of the element.
The atomic mass is listed beneath the element symbols. This is the sum of the protons and neutrons found in one atom of the element. It is an average of all possible isotopes, so, round the value to the nearest whole number to find the most common mass.
Subtract the atomic number from the atomic mass. The difference is the number of neutrons found in one atom of the element.