The shape of the molecule depends on how many electrons and bonds you have in the outer shell of the central atom.
For NH3, N is the central atom. It has 3 covalent bonds and a lone pair. Thus the repulsion of the lone pair electrons and the bond electrons will force them to arrange at the corners of a tetrahedral. The molecule will look as a trigonal pyramid (since the lone pair is not "visible" in the structure.
For BH3, B is the central atom and has no lone pair apart from the three bonds. So the repulsion of the bond electrons will force them to be in a planar trigonal arrangement.
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