Describe how is the atomic mass of an atom is calculated.
urgent, need to know !

If it's your first encounter with chemistry, your instructor will want you to learn how to use the periodic table to find the atomic mass (atomic weight) of an element. This number usually is given below an element's symbol. Look for the decimal number, which is a weighted average of the atomic masses of all the natural isotopes of an element. 

Yes you are correct use the scientific method