Dear Student,
There are many ways of finding the hybridisation -by using the formulas given in the other answers,or by another very fast method.
Just count the number of bonds and no.of lone pairs given in the central atom, and count in the following way: 1 bond count 's', 2 bond count 'sp',3 bond count 'sp2', 4-bond count 'sp3', etc... and counting the lone pair as a bond.(note. +ve charge means that deficiency of electrons,so just count no of bonds.......and one -ve charge is equivalent to a lone pair....so count accordingly)
eg NH3--3 bond and 1 lone pair. so equivalent to 4 bonds Therefore hybridisation is sp3 ..... this method is fast and efficient.
Good Luck