Here's how to calculate it:
1. Gather information:

You'll need the atomic masses (in amu) of each isotope of the element.

You'll also need the natural abundance (as a percentage) of each isotope. This represents the percentage of atoms of that isotope found in a naturally occurring sample of the element.
2. Convert abundance to decimal:

Percent abundance is usually given as a percentage. You'll need to convert this into a decimal for the calculation. Just divide the percentage by 100 (or move the decimal two places to the left).
3. Weighted average calculation:

For each isotope, multiply its atomic mass by its decimal abundance.

Add the products obtained for all the isotopes of the element.
Here's the formula:
Average atomic mass = (mass of isotope 1 x abundance of isotope 1) + (mass of isotope 2 x abundance of isotope 2) + ... (and so on for all isotopes)
Example:
Let's calculate the average atomic mass of chlorine (Cl), which has two main isotopes:

Chlorine35 (Cl35):

Mass = 34.96 amu

Abundance = 75.76% (or 0.7576 in decimal)

Chlorine37 (Cl37):

Mass = 36.97 amu

Abundance = 24.24% (or 0.2424 in decimal)
Calculation:
Average atomic mass (Cl) = (34.96 amu x 0.7576) + (36.97 amu x 0.2424) = 26.72 amu + 8.94 amu = 35.66 amu
Therefore, the average atomic mass of chlorine is approximately 35.66 amu.
Tips:

You can find atomic masses and isotopic abundances in various chemistry resources, including periodic tables (sometimes listed), handbooks, and online databases.

If you're unsure about the number of isotopes for an element, refer to a periodic table or chemistry reference