Engineering is a subject matter. The people who work in that area are engineers. So a human can't BE civil engineering, but can be a civil engineer. You can say you WORK IN civil engineering however.

The most native-sounding / natural-sounding options: - I am a civil engineer (best) - I work in civil engineering - I work as a civil engineer

I am a civil engineer, or I am in civil engineering.

I am a civil engineer. I am studying engineering. I am doing civil engineering.