I don't know about you but as a child, I had a diary with a lock and key on it where I would write silly things that happened to me in the day, whether it was about the cute boy in class or my sister annoying me. As I grew up, that diary slowly turned into a journal where I could keep track of my thoughts, emotions, and feelings. Especially as a teenager going through puberty, I had a lot of thoughts and feelings that I didn't understand along with the stress that comes with high school and college.
Journaling has a lot of benefits. It allows you to express yourself and it helps with mental health. Some benefits include reducing stress and anxiety, improving your mood, tracking symptoms daily so you can recognize your triggers, realizing any fears/problems, and positivity. When writing in your journal, which can be a physical one or online too (I like using notes on my laptop to write down my thoughts), it can help identify what's causing your feelings and can help you realize how to fix those problems. Additionally, sometimes it can be hard to talk about things but writing those thoughts down can help lift a burden off your shoulder and help you feel liberated.
There's no wrong way to journal either! Make it easy for yourself and write whenever you feel like it and whatever you feel like writing about. You don't have to share it with anyone either - it can be for yourself or you can share it if you feel like it.
Keeping a journal / a written record about your thoughts can help you process your emotions that we feel as humans. Sit down in a quiet place and just let it all out! I hope this helps to process your thoughts and keeps your mental health in check.