Steve Marsden’s

It is actually easier to start with a later model proposed by Gillespie and Nyholm.

Valence Shell Electron Pair Repulsion Theory (VSEPR) is a model for determining molecular geometry which relies on one simple principle: electrons repel one another. So in every molecular structure the electron pairs on the central atom, whether shared in bonds or unshared ("lone pairs"), will distribute around the central atom so as to be as far apart as possible. All of the molecular shapes we observe can be rationalized from this simple premise.

It's important to keep in mind that it is the electron pairs on the central atom that are doing the repelling and generating the shape, not the electrons on the atoms attached to the center.

For most common molecules the possible geometries will fall into one of five major shape categories, based on the number of electron pairs on the central atom. You need to know these.