Fully agree with the parent comment about it being like clearing out RAM!
I have also ordered an A6 "pocket" notebook where I want to write one or two sentences or just key words per entry as condensed conclusions to my thoughts or sayings or quotes i heard or read. It should be this condensed book of knowledge and philosophy where long form is not exactly suitable for it.
PS: in my first month, i wrote a lot because there was a lot on my mind. But after a while your brains gets a bit quieter because you are doing this "exercise" and you will go through a short phase where you will be looking for something to write about. That might last a week or two until you settle into the habit and write only when you feel like it. I kinda miss writing when I have nothing to write about so sometimes i just write meaningless thoughts just for the sake of writing. Not all the time, just sometimes.
Journaling off and on since 1987, I have a few rules for myself when journaling:
1. Write the time, day of week, date, month, and year for every entry.
2. Don't worry about lapses in writing. Just write a new entry when I can.
3. Allow my topics to be spontaneous. I sometimes have a structure I'd like to use, but allow freedom in any moment.
4. Relish mistakes, smears, stains on the paper by labeling them, e.g. "tea stain while studying with Akane" (real example!)
5. Just enjoy the process of writing.
As to what you ought to write, maybe start by transcribing some of your favourite poems, song lyrics, quotations, etc. Don't wait until you have some "big thoughts" that are "worthy" of a fancy notebook. Your first step is to see if it feels lie fun to you.
I also keep a journal, but I do that on computer because I value being able to electronically search it.
Shopping lists, random thoughts, UML-like diagrams of code, sketches of possible art pieces, knots, etc. Just anything.