Here's how you could do that.
To keep the answer at a reasonable length, I"ll assume that you know how to draw the Lewis structure for thionyl chloride,
The molecule has a total of 26 valence electrons, 6 from sulfur, 6 from oxygen, and 7 from each of the two chlorine atoms.
Now, let's use the structure on the right, the one in which sulfur is double-bonded to the oxygen atom.
In your case, sulfur is surrounded by 4 regions of electron density, one lone pair of electrons, two single bonds and one double bond. Remember that single, double, and triple bonds all count as one region of electron density.
The number of regions of electron density is actually called the steric number and will help you determine the hybridization of the central atom.
More specifically, the number of hybrid orvitals is equal to the steric number. This means that sulfur will have 4 hybrid orbitals, which is equivalent to saying that it is
Since the central atom is surrounded by three other atoms and one lone pair of electrons, the molecular geometry will be
The lone pair will occupy more space around the atom than the bonding electrons, and as a result will push down on these bonding electrons. This implies that the molecular shape will be trigonal pyramidal.