The tone of the title is weird since I am angry while writing this but this kinda needs to be done and written down.
The journal will keep you from getting misinterpreted and lied about by whoever finds it. If you don't have any idea what to write you can just put your imediet thoughts on paper.
By everything I mean everything. Your favorite books, games, video essays, that cringe fanfic you wrote when you were 12. Also have phisical copies of them because you don't know when there's gonna be a blackout or your harddrive won't work anymore for whatever reason.