O.K., you want help but you just said you "hate history." Now, that would make me REALLY want to help you. Despite this remark, I have the following ideas:
1,. Do you have some books on the Civil War? If not, I would start there. Perhaps a general history that lists the causes, etc.
You would also want to make sure you have a book that has the major battles (at least) in it.
2, Decide if you want a real person or make one up. My suggestion is to make one up. You have more freedom.
3. Decide if your person is male or female. If it is male, you can put your character into the battles.
4. Read some diaries of actual soldiers who lived during the war. Their entries should give you an idea about what to do.
5. If you can, get the videos/dvds of Ken Burns' Civil War. This is an excellent series on the Civil War. Portions of the video contain diary entries (read by celebrities and historians).
6. Set up your outline. When do you want to start the diary?
At the beginning of the war? Before the war? In the middle?
Then, start- using a calendar (don't jump around). For example if you start in February- don't go back to January in your diary.
7. You can include some "funny" stuff too. It doesn't have to be all serious. For example, if your character is a city boy, you can put him in the Pennsylvania countryside for the Battle of Gettysburg. You can make entries about farm animals and how he reacts to a bull on the battlefield for example.
8. Ask a parent or grownup to check your entries for spelling and content.
You can have fun with this. This seems like a fun project- it's better than memorizing dates for a test.