The movie is about an ordinary man (probably african-american, I don't remember exactly) who becomes a cop suddenly. Movie then proceedes with him wandering around at his night shift, then funny things happen because of his inexperience. This is all about the first 20-30 minutes of the movie. I don't remember anything else. I have searched about 100 titles in IMDB, unfortunately cannot find it.
Important facts about a movie, briefly;

Guy becomes a cop
Genre is Comedy
Most likely `80's or early 90's movie
Most likely movie from the US

Could it possibly be any of the police academy movies?
Police Academy (1984) Police Academy (1984) (http://www.imdb.com/title/tt0087928/)
There were 7 movies

It could be (national security) it sounds similar

Can't be national security- that's 2003 and they want a movie from 80's- 90's