Your question is not entirely clear... are you asking how to insert comments
into a cell (that is the question the other posts have answered) or are you
asking how to control the display of the "red triangle" when a comment is
present in the cell? If the later... click the Office button (the largish
round icon in the top left corner of the Excel window), click the Excel
Options button at the bottom of the display panel that appeared, select
Advanced from the listing on the right side of the next display panel that
appears, scroll the right list down to the section with the heading
"Display" (it's about 1/3 the way down) and at the bottom of that section
are the three options you can choose for comment and the comment indicator.
By the way, the red triangle appears in the upper right corner of the cell,
not the upper left corner.