Each element on the periodic table includes the atomic (or proton) number, which tells you the number of protons in the atom, and the atomic weight, which tells you the weight of the atom, as shown below for hydrogen:
The proton number will always be an integer, which is an easy way to remember how to tell them apart.
To determine the atomic weight of an element, you simply find it on the periodic table, and read the atomic weight.
Alternatively, you may want to calculate the atomic weight using the different isotopes and their abundances.
Carbon-12 has 99% abundance
Carbon-13 has 1% abundance
So the atomic weight of carbon is: