How can you tell what shape a molecule is going to be?

The key to determining the shapes of molecules is to think about number of lone and bonded pairs of elections and the interactions between these pairs. In doing this we apply the valence-shell election-pair repulsion (VSEPR) theory. In summary, this theory states that all electron pairs repel each other, so spread as far away from each other as possible to minimise repulsions. 

The other point to consider is the strength of these repulsions, lone pair-lone pair repulsions are the strongest, followed by lone pair-bonded pair and bonded pair-bonded pair is the weakest. So by counted the number of bonded and lone pairs of electrons we can figure out what shape a molecule is going to be. (After and during this I would demonstrate with a few example diagrams and then some questions to check understanding).