The mass of anything in atomic mass units is the atomic mass from the Periodic Table.
The mass of Carbon on the Periodic Table is 12.0107 amu (atomic mass units), but if you are looking for the APPROXIMATE mass, then it would be 12 amu.
For any molecule, like CO2 (carbon dioxide), you just add up all of the atomic masses of the atoms. Carbon is 12 and then each Oxygen is approximately 16 (15.99 amu), so 12 + 16 + 16 = 44 amu total. :)