Answer:
You can write diary entries however you like! Diaries are supposed to be a space where you're comfortable sharing, and part of this is writing it in a way you're comfortable.
However, a lot of people structure them with the date and topic at the top. You could include your mood on that day, your plans, or activities. In the body paragraph(s), you can write out what happened on that day, things that bothered you, or things that you enjoyed.
Again, there is no right way to go about writing diary entries! The nice thing about diaries is that they don't have to be formal or structured at all. Do whatever you like and structure it however you like.