N₂ + 3H₂ →2 NH₃
This would be your equation; N and H are diatomic molecules so the reactants would be N₂ and H₂ the when they react you have to take into account the charges on each atom so N has a charge of -3 and H +
so your product from combining N₂ and H₂ becomes NH₃
Then make sure the equation is balanced (has equal number of each element on both sides)