To determine the shape you need to start by drawing the lewis dot structure, then count the number of electronic domains, and determine the type for
When you draw the lewis dot structure of
Every molecule you can break down in to a
Then you need to just memorize the structures for the different types.
It is easiest to just start with the structures with no lone pairs.
Keep in mind that that structure the molecule takes, is one that will minimize the amount of electron - electron repulsion .
A table with all the possible structures is given here: