In order to do this calculation we first need to understand the concept of the mole. Just as a "dozen" of something means 12 items, or a "pair" means 2, in chemistry a "mole" of something means you have 6.02 x 10^23 items.
When you look at a periodic table of the elements, you see that sodium has an atomic weight of 23. This means that a single atom of sodium weighs 23 atomic mass units (AMUs), but it also means that one mole of sodium atoms weighs 23 grams. Hence 6.02 x 10^23 atoms of sodium weight a total of 23 grams. This means that if we divide 23 grams by the number of atoms, we should have the weight of a single atom.
23 / 6.02 x 10^23 = 3.8 x 10^-23 grams
So one atom of sodium weighs 0.000000000000000000000038 grams