Samuel Pepys was a civil servant in late 17th century London. He kept a diary for ten years. His daily entries recorded who he met and who he entertained, his conversations with his wife and his noting of business, good food and theatre visits. His record of his everyday life allows us to have a picture of the life of a government official at the time and something of London society.
Here is his entry for Saturday 9th January 1668/9:
Up, and at the Office all the morning, and at noon, my Lord Brouncker, Mr Wren, Joseph Williamson and Captain Cocke, dined with me; and, being newly sat down, comes in, by invitation of Williamson’s, the Lieutenant of the Tower, and he brings in with him young Mr Whore, whose father of the Tower, I know. — And here I had a neat dinner, and all in so good manner and fashion, and with so good company, and everything to my mind, as I never had more in my life — the company being to my heart’s content, and they all well pleased. So continued, looking over my books and closet till the evening, and so I to the Office and did a good deal of business, and so home to supper and to bed with my mind mightily pleased with this day’s management, as one of the days of my life of fullest content.
This website publishes a daily entry from Pepys’ diary which includes links to further information about him and his time.
Joseph Williamson (The Diary of Samuel Pepys) (pepysdiary.com)
There are plenty more diaries to look at for style and content. Star with Pepys, if you wish, or another diarist whom you prefer. Have a look at what the writer decides to record and to what extent the diary is written in note form. Pepys includes incidental pieces of explanation and often expresses his opinions and moods. Although it may seem to us that we have nothing to write -each day is ordinary and much the same as the day before, our ordinary day is unique to us and has the potential to be interesting to others in the future.

Imagine how interesting it would be if your parents and grandparents had kept diaries at your age. Perhaps they did. You might find that there were things that were different about their lives and what hey felt was important, but you might also find things that are very much the same.
Write the date at the top of your page and write you diary entry.