The mass of an atom is known as the atomic mass. The atomic mass of an atom is the sum of the protons, neutrons, and electrons for an atom. However, the mass of electrons is so small that it is considered insignificant towards atomic mass. Therefore, to determine the atomic mass of an element, one would simply add the number of protons and neutrons housed within the nucleus of that element.
The atomic mass of an element can be found on the Periodic Table. Most often it is found directly under the symbol of an element. This value is the number that contains decimals. This is the weighted average of all the atomic masses of the naturally occurring isotopes for that element
Sometimes you are given a set of isotopes, their masses, and their relative abundance (either as a decimal or a percent value). To find the atomic mass when given such information, you would start by multiplying the atomic mass for each isotope by that isotope's relative abundance (in decimal form- thus, if given percents, you would have to divide each by 100). Then, add each of these values to find the atomic mass.