Hybridisation = number of sigma bonds + lone pair.
CH3Cl = 3 sigma bonds between C & H and 1 between C and Cl
There is no lone pair as carbon has 4 valence electrons and all of them have formed a bond (3 with hydrogen and 1 with Cl).
Now, hybridisation = (3+1) + 0= 4
= sp3 (1 s & 3 p).