To answer this question effectively you need to focus heavily on what happens in Act I scene 2 and to imagine what Miranda would feel like after having been told the true story of her own background and how her father and she came to end up on the island that is the only home she ever remembers. It is clear from her reaction to what her father tells her about how they were exiled and sent to this island that she is emotionally impacted, as the following quote demonstrates:
Alack for pity!
I, not rememb'ring how I cried out then,
Will cry it o'er again; it is a hint
That wrings mine eyes to't.
Your diary entry will therefore need to capture some of this emotion and sorrow and sadness. Although Miranda is not able to remember specifically the events and the details about her past and how she and her father were forced to leave Milan, where he was Duke, it is clear that her father's retelling of this event causes her to feel considerable sadness and grief about what happened to her. Your diary entry therefore needs to include a summary of this background, and also her feelings about what happened. You also would benefit from thinking about what kind of questions Miranda would have after this. It is clear that whilst Prospero is willing to tell her a certain amount, the way that he charms her into an enchanted sleep when she asks about the purpose of the tempest he has summoned indicates he intends to keep her in the dark with regard to some information. Miranda must have been incredibly curious about why her father told her about his past at this particular point, and what the purpose of the tempest was. Indicating this in your diary entry will help you to get good marks.