Calculating the charge of a molecule e.g In NH4 what is the charge of the nitrogen atom?

For the atom which you want to calculate the charge determine the number of valence electrons a free atom usually has. this is the number of atom on the outer shell so for nitrogen, it has 5 valence atoms .

For the charge of the atom in the molecule calculate the number of nonbonded free electron and half the number of shared electron. nitrogen does not any nonbonded electrons and has 8 shared electrons as it is bonded to 4 hydrogens so 8/2 = 4 

Take the difference between the free atom and the atom in the molecule. if the number of the free atom is bigger than the in the atom in the molecule then when the atom charge is positive by the difference. if the number of electrons in the atom in the molecule is bigger than the number of the free atom than the atom charge is negative by the difference.  if there is no difference, then the atom has no charge. For the nitrogen, the number of electron in the free atom is higher than in the molecule and the difference is one. so there is one positive charge on the nitrogen in ammonia.