How to Find the Most Common Isotope
Find the element on the periodic table. Record the atomic weight (on the bottom) and the atomic number (top left).
Round the atomic weight to the nearest whole number. If the decimal is .5 or higher, round up, if it is .49 or lower, round down.
Subtract the atomic number (the number of protons) from the rounded atomic weight. This gives you the number of neutrons in the most common isotope.
Use the interactive periodic table at The Berkeley Laboratory Isotopes Project to find what other isotopes of that element exist.