What happens at the start of a film?

http://www.artofthetitle.com/title/21-jump-street/

In the begging of the film 21 jump street i could visualise...

• The main characters (Channing Tatum, Jonah Hill)
• Guns/Police/Cars
• Parties/Drugs 
• Explosions
• What the film is going to be based on (Police/Law)

The begging of the film allows us to understand the main characters and what they would basically be doing throughout the film which is to do with police. The music allows us to understand that the film would be based around teenagers as the music connotes young and immature which gives the audience a basic idea of what the film is going to be about. The film showed several scenes of drugs which would be a stereotype of young people using drugs such as Marijuana. It also shows us who has directed, edited and published the film.