nitrogen has atomic no 7 i.e. it has 7 electrons of which there are 5 in outermost shell.
in case o NH3, N will share 3 of its 5 outermost electrons and there will be one lone pair.
therefore NH3 will have steric number=4(3 sigma bonds+1lone pair) sho hybridisation=sp3
so if we consider its 3d shape there will be lonepair on one side and onthe other there will be the three h atoms this because of the lonepair-bondpair repulsion. of the 3 hydrogen atoms one will be on the plane, one outside the plane and remaining inside the plane.
so NH3 will have a trigonal pyramidal shape and geometry