"The life of every man is a diary in which he means to write one story, and writes another; and his humblest hour is when he compares the volume as it is with what he vowed to make it."
That's a rather weighty thought, but the fear of not living up to the best version of one's self shouldn't intimidate anyone who wants to be a diarist. It's interesting to compare your real life to your ideal version of it, and it's hard to do that without a regular account of what that real life is like, but that's not the point of the project. You don't necessarily need to fill pages with deep thoughts to make it worthwhile--your everday activities and whims are enough. These things might seem dull or too common in the moment, but the most valuable record for historians turns out to be a diary of the mundane. The things no one writes down because everyone already knows them get forgotten over time, and then we lose track of what life was like in other times and other places. So just write whatever comes into your head, and be inspired by these many different records of many different lives.