In short, you should put whatever is important to you. And whatever you think you will want to keep and look back on later. It is your journal and it is up to you to decide what that is. What is important to someone else may not be to you and what is not important to them might be to you.
Mine has all my readings, I personally number the pages and put an index at the front. For myself I have found it useful to keep past readings for two reasons. First sometimes in the beginning you get cards that don't make sense. If you keep the reading, and look back at it months later, you often (usually) see how the cards that came up were perfect there. I also get a kick at looking over old readings to see how accurate they were. You can learn a lot from doing that. As well, I like to look over successive readings on a situation to see the progression of it.
I also keep any other articles of a spiritual nature that I feel I will want to look at later. I have keep spread I might want to try later in my journal, techniques for timing questions, even how to crochet a Tarot pouch. I also tend to keep readings others do for me. I also save readings done for me there.
Although to tell you the truth my Tarot journal pretty well contains any spiritual pursuit I am into currently. It also have Rune readings, Oracle readings, pendulum readings and most recently a collection of articles of methods of co-creation.
Hope you're not even more confused now, LOL
Babs