When I journal, I just write what's on my mind at the moment. It doesn't have to be organized or written like a book. It doesn't need an intro or a story. As long as I'm putting my thoughts down, then things will start to flow onto the page. Don't try too hard or judge what's being written. Your words don't have to be "right" because it's really just about exploring your thoughts and feelings. It takes practice to write, but nobody else is watching and nobody else will have to ever read it, so just start.
Anything! The first rule is that your journal is your space and you can do whatever you want with it. Here are some suggestions though. You can write down what happened that day, negative emotions you want to exhaust or positive emotions you want to record, alternatively you can find an online prompt to follow, or use one of the fabulous 'sit andthink' exercises,any of which are designed to be done with a journal. Even doodling is an aceptable form of journalism; I bulletjournal, so for me I have a mix of daily planner, to do list, journal and sketchbook all in one.