how to auto calculate difference between cells like c1=a1-b1

[Jill]

how do I set up Excel spreadsheet to auto calculate the difference , like
c1=a1-b1, I want the whole column to do it, so when i enter values in like
a1,a2, a3, etc, it automatically calculates the difference.

 

[Gary''s Student]

In C1 enter:
=A1-B1

copy this cell and paste it down the column.

 

[Gaurav]

in C1, type =A1-B1 and drag it down as far as needed.