Act One lays the foundation to the reality of the Loman family. It is here where we witness the Loman's the way that they are "at present".
The dynamics that we witness may lead us to perceive the Lomans as a very enmeshed family, that is, a family whose members are too dependent on each other for either financial, physical, or psychological reasons. We learn that the latter is what actually fits the Lomans, for they are a family whose leader, Willy, has lived a life of mere fantasy. He has chased the wrong dream his entire life and has dragged his family along his useless journey.
Willy's entrance in the play uncovers the dangerous nature of his behavior. We know that he has crashed his car many times, that his mind wanders, and that he tends to lose touch with reality. These actions are actually normal for Willy. At least, they have become normal enough for him to casually say that he nearly "hit a boy with his car in Yonkers". That is pretty weird when you consider the consequences of what may have occurred. It also demonstrates the dire state of Willy's mind.
Linda comes across almost immediately as the enabler. She babies Willy, feels sorry for him, looks after him in every way and, quite annoyingly, keeps asking him whether he wants this or that. She offers him an aspirin, water, and food as if Willy were unable to fetch things for himself. Moreover, she basically prepares him to interact with their two sons, who are staying over. It is as if Linda has awarded herself the role of a "referee" in the family; a type of regulator among the men. This is ironic considering that Linda, although she is respected in the family, has never been treated with the same enthusiasm as she bestows upon the others.
Finally, we see that Happy and Biff are basically two little boys trapped inside the bodies of two grown men. Happy even admits to that, by calling himself "a boy", when he speaks about his inability to settle down and find a purposeful job. Similarly, Biff shows that he is likewise lost. Biff comes across as the biggest victim of Willy's mentality because we see how, in Willy's flashbacks, he places Biff on a high pedestal; Willy lives vicariously through Biff and this will result in their final fallout.
Therefore, Willy seems to have boxed his entire family inside a fantasy world, thus rendering them unable to grow and diversify their lives. It is as if they all need Willy's permission to move on. This is understandable, however, when your family is enmeshed and dependent on one very confused leader.