To balance this equation, write down the number of atoms of each element in both left and right side of reaction.
NH3 -> N2 + H2
N=1 N=2 H=2
H=3
Then basically place appropriate coefficients in front of the reactants and products to have the atoms of each element equal the same on the left and right side of the reaction.
2NH3 -> N2 + 3H2
N=2 N=2 H=6
H=6
This equation is balanced.
