First of all count the total number of sigma bonds and the total number of the lone pair of electrons on the central metal atom. Add them. The answer obtained will decide the hybridization.
For example in NH3. Central metal atom is N with 5 outer most electrons and 3 of them making 3 Sigma bonds with 3 H's and Thus leaving one lone pair of electron.
so No. of Bond Pairs + No. of lone pairs = 3+1= 4
4 denotes sp3 hybridization. i.e 1s + 3p prbitals