Today I did blah blah blah and explain what you did in chronological order.
You can also add feelings and emotions that you are feeling, drawings, observations and maybe write a timetable of your day/week (depending on how busy you are).
It shouldn't just be everything you did that day, just the most significant. Maybe you can write about a dream, a secret, a crush etc. Some people choose to put things in their diary such as amusment park bracelets, movie tickets, programs from school shows and concerts and even notes they passed in school or doodles they drew during class. It's your diary. Make it about you and you only. Act as if your diary is a world where you are the queen/king and everyone worships you.
You might write about what you did in school, at home, etc. You may pretend you are a prince or princess and write about people you wish were your friends or lovers. Or it may be a wish that a loved one was still alive.