How NO2 is obtained?

If you are talking about Nitrogen dioxide, the process for obtaining it is quite simple, but dangerous.

It can be synthesized by reacting concentrated (about 64%) Nitric acid with a metal like Copper, Zinc, etc .

It appears as a red gas in room temperature but it is quite toxic. It is denser than air and is also an air pollutant. It is not stable in negative temperatures, at which it dimerizes into Dinitrogen tetroxide, a yellowish gas