Journalling is like a diary in some ways. My first attempt at journalling was during my first daily Bible study right after I became a Christian. I wrote down what chapters I had read and then wrote comments about what I learned, or thought about, while I read the verses. Sometimes I wrote out my prayer to God and things I had asked Him to do in my life. I had that I wanted to think about more later.
Writing can be good therapy.
When I simply think about a problem, I tend to recycle the same problem without any insight. I have a couple of blank books that I can write my deepest hurts in, and when written, I give it to God, then put the book on the shelf. Later when a similar hurt erupts, I can go back and read about the other event and look for patterns in my behaviour and how I contribute to my own pain. That helps me know what changes to make. A journal helps me remember when events happened and how I felt about them.