Shapes of Molecules

When working out the shape and bond angle of a molecule, I know you need a central atom which is bonded to other atoms e.g. NH3. But when the molecule is something like (CH3NH3) +, how do you know which one the central atom is?? and how would you find the shape and angle?

It does not make any difference provided that you state which angle you are discussing. There are two nuclear centres, each with its own bond angles.

Carbon is tetrahedral with HCN bond angle equal to 109.5º
Nitrogen is tetrahedral with HNC bond angle = 109.5º