The place I would start would be to examine very carefully what the text tells us about how Biff thought about his father both before and after his discovery of the Woman and his father's infidelity. This was, of course, the big breaking point for Biff in his life as after this he gives up all intentions of retaking Math and going to college, and instead spends his days itinerantly wandering from farm to farm, working and feeling dissatisfied with his life. The various flashbacks that the text presents us with before this time show Biff following his father around like something of a god, hanging on his every word and desperate for his approval. A few metaphors and similes might be a great idea to capture that relationship, and I have just used two in the previous sentence that you could pick out: "like a god," "hanging on his every word."
As for Biff after the discovery, you might want to take a specific concrete section of the play and use that to base your diary entry around. Note what Bernard says to Willy in Act II about Biff when he came back from Boston:
And he came back after that month and took his sneakers--remember those sneakers with "University of Virginia" printed on them? He was so proud of those, wore them every day. And he took them down in the cellar, and burned them up in the furnace.
The sneakers are clearly a symbol of Biff's hopes to go to college, and therefore his burning of them at this stage is symbolic too of the loss of that dream. Your diary entry could explore this particular incident, describing how he burnt them and his feelings as he watches his sneakers--and his dreams--go up in smoke. Pointing out the symbolism in this way would be another great technique to use that shows your teacher you have picked up on some of the literary techniques in the text. Good luck!