First i want to say that hybridisation is a hypothetical concept. many this occurs when to orbitals overlap each other i will explain this whith a nice example
   Let us consider a molecule methene the structure is
                                                  H-H-C=C-H-H (acutually the hydrogens both are atached to carbon only due to the computer typing the structure is not clear )
  So, the carbon as one double bond amd only two hydrogens atached to it so its valency is four and the atachaments are four so its having a double bond so it has SP2 hybridisation