In group, we had to write down our current feelings and how our days went, what our mood was like and triggered by--whether it be deemed good or bad. Basically what Shattered wrote pretty much.
I Googled some writing prompts for mental health and self reflection. This caught my attention, just because it's on the more crafty side of things.
Don't get caught up in trying to make everything in your journal perfect. Just let the words come and flow. You'll get the hang of it.