Question

how to calculate dipole moment

Answer 1

So, the dipole moment is used to show the positive and negative atoms in a molecule. You would look on the periodic table to find out what the charge of each atom was, and then draw an arrow that points from the positive atom, to the negative one on that molecule's Lewis Dot Diagram.
Here's an example with hydrogen fluoride (It's kind of hard to type it in this box...):
..
:F :H
˙˙
<---+
So you see the lewis dot diagram, and the arrow is on the bottom with the plus on the positive end.

Answer 2

Dipole moment = Magnitude of the charge × Distance between the charges
Using the above formula, Dipole moment can be calculated.