How to execute my trigger when update certain columns but not all?

Let say I have a table has column1 and column2 and I made a trigger for
after INSERT OR UPDATE for each row to execute procedure my_function.

What I want is the trigger execute my_function only when column1 is
insert or
update, but not going to execute my_function when column2 is insert or
update.
How do I do that?

Thank Q very much