(a) Valence Shell Electron Pair Repulsion (VSEPR) theory says If a  pairs of electrons that surround the central atom of a molecule or ion are arranged as far apart as possible to minimise electron-electron repulsion.This  idea will  helps to predict the shapes of molecules by using a small trick: First find the central atom in a molecule. In cases if you are confuse, pick the least electronegative atom as this atom will be better able to share its electrons with the other atoms in the molecule.

(b) Count the number of  valence electrons on the central atom.

(c) Count up the electrons used by the outer atoms to make bonds with the cental atom.

The sum of (b) + (c) divided by two gives the Valence Shell Electron Pair (VSEP) count.    

The predicted geometry of the molecule is based on the number of VSEP.  VSEP Shape is as given below

2  -  Linear

3 -   Trigonal Planar

4 -   Tetrahedral

5  -  Trigonal Bipyramidal

6  -  Octahedral

Example