The First Act of The Graduate

The Graduate was filmed in a classical 3-act structure. In the first act of the Graduate, we are introduced to the protagonist Ben. He walks through the airport, on his way home. He arrives at home, somewhat displaced. He has just graduated from college, and was uncertain of were his future was going. The viewer can sense this through camera placement, and Ben's awkwardness and need to be alone. What seems to be out of the blue, Mrs. Robinson asks Ben for a ride home. To Ben's dismay, when they arrive, Mrs. Robinson comes on to him. Mr. Robinson comes home establishing a second antagonizing presence.
Out of loneliness Ben calls Mrs. Robinson, and they end up having an affair. So far, two of the main characters have been introduced. Ben's struggle with his future, his affair, and his lack of directions are all established in the first act. He is reaching for something to distract him from his conservative life, no matter how hard he has been trying to uphold his family's values. It just so happens that Mrs. Robinson is the first goal he has tried to obtain in the film, when he has been virtually drifting without purpose so far.
I believe the plot point of act one, that leads into act two,  occurs at 00:24 minutes into the film when Ben decides he will call Mrs. Robinson to meet him for their first affair leading to the peak of the conflict of the first act. He is clearly uncomfortable about doing this, but it shows that he is attempting establish some kind of goal. He isn't sure what that is, but he confronts the situation in attempt to create some kind of substance in his life. You could also interpret that he was bored, lonely, and horny, and seeking some kind of action in his hum drum life.
The creative use of music, camera placement, and great acting made this film history. While it is in a classical three act structure, there are instances of new and creative film techniques and frame usage. The first act is always the most important in laying the ground works for the ultimate resolution and climax of a good film.

http://www.imdb.com/title/tt0061722/