Nitrogen gas reacts with hydrogen gas under high temperature to form ammonia gas.

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)