How do you calculate the number of protons, neutrons, and electrons in an atom?

First you need a periodic table of elements.

Then you look for your element. Let's go for Krypton as an example.

Krypton has an atomic number which is on the left upper corner and an atomic weight which is at the bottom of the element.

To find protons it is the atomic number. The atomic number for Krypton is 36.
To find electrons it is the same number as protons. 36.
To find neutrons you use the atomic weight which is 83.80 and you round it to the nearest whole number which will be 84 and you subtract it from the protons. The answer is 48. 

Protons = 36 (atomic number)
Electrons = 36 (atomic number)
Neutrons = 48  (atomic weight round to nearest whole number - atomic number)