Write about what their life was like for them, before the story you have in mind takes place. This gives you a chance to see sides of your character that you do not get to see in the story. You can write a little about their history, feelings and thoughts. You can weave their history in the diary the same way as you would in the story. Or, is it the date that was their first date with the guy who later broke her heart.
Feelings come with writing the dairy.
The diary is their thoughts. This will help you find the voice you want to use before you even start your story. It will help you get an idea of what kind of character they are. Write the dairy as your character for a week or so. Then go back as se if you have learned anything new about them.