Most Helpful Girl
Get any writing app or a book depending on which suits you best. Write single words or a phrase as you go and expand when you have time.
Most Helpful Guy
I just use a word document. I've been doing it since 2010. Whenever I'm bored or need to vent, i just open it up and type whats in my head. I'll talk about plans, things that annoy me, my worries, etc, I just let it all put.
I mark each month with a header and then each days with the day of the week, day of the month, month, and year (e. g. Saturday 8th December 2018).
You can also put passwords on word documents. That way you can feel more secure when writing anything down.