Sulfur is the central atom:

There are 3×6 + 6 + 2 = 26 electrons, and 6 of them are used to make 3 bonds. Three oxygens take 3 lone pairs each and one goes to the sulfur:

All the electrons have been used; however, one oxygen needs to share a pair of electrons with the sulfur otherwise the ionic would be -3:

The central atom has 3 atoms and a lone pair and therefore, the electron geometry is tetrahedral while the molecular geometry is trigonal pyramidal.

Steric number 4 corresponds to sp³-hybridization where the idealized bond angles are 109.5^o. Because the groups on the central atom are not identical, the bond angles differ from the idealized values.