Take acting seriously. Some people are only interested in acting for the money and fame, but to Marlon Brando, acting was very personal. He came from an abusive and poor family, so acting was the one thing that he felt good at, and it made him happy. If your goal is to become as legendary an actor as Brando, acting has to be an essential part of your life.
Study the art of acting. Take classes in acting for theater as well as for film and television. Marlon Brando had a thriving stage career before he finally hit the big screen. One of his greatest roles was as Stanley Kowalski in both the theater and film productions of Tennessee Williams' "A Streetcar Named Desire."
Focus on the Method. Method acting is a common form of acting today, but in Brando's time it was seen as radical. Many classic actors were stage veterans whose over-the-top mannerisms transferred well from theater to film. On camera, it looks as if they are exaggerating every move and word. The Method focuses on "being" the character, inside and out, living that life, allowing the acting to come across as genuine, rather than rehearsed.
Research films. Watch Brando's work as well as actors who followed in his footsteps like Dustin Hoffman, Al Pacino, Robert De Niro, Sean Penn, Johnny Depp and Leonardo DiCaprio who all used the Method. Watch footage of them in an interview as themselves, then watch their films. You can see how they change so drastically to become their characters, and you really believe they are that person.