It will be much more easy if you consider the lewis structure before you predict NH3's shape.
There're 3 single bonds in NH3, while N is the center of all, with a lone pair.
and by VSEPR, NH3 is tetrahedral.
Therefore, there're 5 electrons available for bonding, which is 2s2 and 2p3.
in order to form a tetrahedral arrangement, there should be 4 orbital around N.
Thus it's a sp3 hybridization as followed:
X(lone pair)
/(overlapped with 1s of hydrogen)
/(same)
/(same)