How can I keep a diary from a main character’s point of view in Animal Farm? What character should I choose?

Snowball would be an interesting choice because he spends part of the novel on the farm and part of it away from the farm. After being exiled, it would be unique to get Snowball's perspective of the farm as if he were watching from afar. In the novel, there is no indication that Snowball is looking on, which is why this perspective would be interesting.

You could write about how he gets inspired by Old...

Snowball would be an interesting choice because he spends part of the novel on the farm and part of it away from the farm. After being exiled, it would be unique to get Snowball's perspective of the farm as if he were watching from afar. In the novel, there is no indication that Snowball is looking on, which is why this perspective would be interesting.

You could write about how he gets inspired by Old Major's speech in Chapter 1. In Chapter 3, Snowball would probably write about his interest in forming committees: 

He formed the Egg Production Committee for the hens, the Clean Tails League for the cows, the Wild Comrades’ Re-education Committee (the object of this was to tame the rats and rabbits), the Whiter Wool Movement for the sheep, and various others, besides 12 instituting classes in reading and writing. 

Snowball wanted to create efficiency through organization and education. In the meantime, Napoleon had no interest in this. By this point, and certainly by Chapter 4, Snowball might share thoughts on how there is something about Napoleon that he does not trust. 

Chapter 5 is Snowball's. His planning and heroic actions lead the animals to victory at the Battle of the Cowshed. 

In Chapter 6, Snowball argues his case for building the windmill. Napoleon disagrees. Snowball is later chased off the farm. Napoleon builds the windmill anyway and then slanders Snowball's reputation in an attempt to turn him (Snowball) into a symbol of a traitor. 

In subsequent chapters, you (Snowball) might keep journal entries while watching Animal Farm from a distance. He would note how Animal Farm has strayed from the original principles they established with their first rebellion. Snowball would write about how Napoleon has become a tyrant. And he would mourn the death of Boxer, who worked harder than anyone to complete their revolution.