"When I look at a product, should I be looking at information on
calories or information on fat in order to determine how much weight
it will add to my body?

Please I need a simple answer on this.  Is the amount of fat already
precalculated into the caloriy number already?"

The calories are most directly tied to weight status.  The calories from
any fat is in the total.  Carbohydrates and proteins have about 4
calories per gram and fat about 9, so something high in fat has more
calories gram for gram.

    There's nothing simple about this question.  ;-)  It depends not only
on your energy output, but whether you are using your body's fuel
aerobically or anaerobically; the amount of glycogen in your muscles,
and many other factors that I've probably forgotten about metabolic
pathways over the past 40 years.
    Add to that the fact that fat slows the transit of food through the
body; has greater satiety value than carbohydrate; and has less glycemic
affect, and you begin to see that this is a complex subject.
    Bottom line--if you eat too much carbohydrate, protein, or fat, you
will probably gain weight.

Steve