How the nitrogen oxide is made ? and How the excessive amount of nitrogen oxide is harmful ? 

At high altitudes when lightnings strikes, dinitrogen and dioxygen combines and form nitrogen oxide.

How the excessive amount of nitrogen oxide is harmful?

Higher concentration of $\mathrm{NO}_{2}$ damage the leaves of plants and retard the rate of photosynthesis.

Nitrogen oxide is a lung irritant that can lead to an acute respiratory disease in children. It is toxic to living tissues $\mathrm{NO}_{2}$ is harmful to various textile fibres and metals.