We can understand the concept of hybridization along with the effect of lone pair on the structure by taking the example of ammonia molecule.
In ammonia molecule, nitrogen has the electronic configuration as 1s2,2s2,2p3 .the p subshell is half filled. Initially s and p orbitals have different energies. But as you know hybridization is the mixing of different types of orbitals to produce the orbitals of similar energy. One s and three p orbitals combine to form four hybrid orbitals hence the hybridization of the molecule is sp3.
sp3 hybridization suggests the structure to be tetrahedral. There are only three hydrogen to overlap with three hybridized orbitals. The fourth orbital accommodate the lone pair of electrons in it. We have learnt that repulsion of lone pair-bond pair is greater than the bond pair-bond pair. The normal tetrahedral angle is 109.28 but due to presence of lone pair, the bond pair-lone pair repulsion leads to a decrease in the bond angle and bond angle becomes 107.5.
Thus we see that if we have 2 hybrid orbitals, the structure is linear, if 3 orbitals then trigonal planar shape, if 4 orbitals then, tetrahedral shape, etc.