What does one journal when one journals? (Or is it, What does one journal when one is journaling?)
Either way, the phenomenon of journaling is a great, promising, it-can-mean-anything new sort of exercise we're all to do but not really know why, how, when, or what it is.
So pick something. Either what you want to do or something you've never done to see what it's like to do it.
Just pick something.
Now, journal away.