I could do this for you but you learn nothing thereby. Open the book, and examine every scene in which Horatio appears. Write down the text of all Horatio's statements. Who is Horatio? A student at Wittenburg, a dear friend of Hamlet's, and what else? What kind of guy is Horatio? Key: just before the play scene, what does Hamlet say to him? Hamlet describes Horatio, and makes it clear why he depends on him.
Where is Horatio at the tragic resolution of the play, and what does he try to do? What job does Hamlet charge him with, before he dies?
"I am Horatio, a very close friend to the late Prince Hamlet. I came from my studies at Wittenburg to Elsinore, to attend Hamlet at the funeral of his father. I found him in a strange state of mind because - - ".
Go for it: if you read the play, you can do this.
Since we are not allowed to do the assignment for I am going to give you some information for you to contemplate as you write your assignment.
Horatio is the very best friend a person could have. He usually goes along with Hamlet and only once does he directly question Hamlet's actions. Hamlet could stand to be a bit more like Horatio, but it Horatio who adores and looks up to Hamlet.
Horatio is intelligent and thinks deeply about the ghost which is haunting Hamlet. Hamlet lets him in on his plot to pretend to be mad so that he can foil his uncle. Horatio sees Hamlet's difficulties in life but sticks by him. Even though hamlet takes his friend for granted, Horatio remains loyal to him. Horatio even offers to tell Hamlet's story so that it will be told with truth upon Hamlet's death.
Try using some of these things to write as Horatio to Hamlet.